diff --git a/cmd/oracle/main.go b/cmd/oracle/main.go
index f7bbce3c..893360e8 100644
--- a/cmd/oracle/main.go
+++ b/cmd/oracle/main.go
@@ -74,7 +74,7 @@ Describe the syntax at offset 530 in this file (an import spec):
 
 Print the callgraph of the trivial web-server in JSON format:
 % oracle -format=json src/pkg/net/http/triv.go callgraph
-` + importer.InitialPackagesUsage
+` + importer.FromArgsUsage
 
 var cpuprofile = flag.String("cpuprofile", "", "write cpu profile to file")
 
diff --git a/cmd/ssadump/main.go b/cmd/ssadump/main.go
index a3b7fd59..28f25d64 100644
--- a/cmd/ssadump/main.go
+++ b/cmd/ssadump/main.go
@@ -48,7 +48,7 @@ Examples:
 % ssadump -build=FPG hello.go         # quickly dump SSA form of a single package
 % ssadump -run -interp=T hello.go     # interpret a program, with tracing
 % ssadump -run unicode -- -test.v     # interpret the unicode package's tests, verbosely
-` + importer.InitialPackagesUsage +
+` + importer.FromArgsUsage +
 	`
 When -run is specified, ssadump will find the first package that
 defines a main function and run it in the interpreter.
@@ -73,28 +73,27 @@ func main() {
 	flag.Parse()
 	args := flag.Args()
 
-	impctx := importer.Config{
+	conf := importer.Config{
 		Build:         &build.Default,
 		SourceImports: true,
 	}
 	// TODO(adonovan): make go/types choose its default Sizes from
 	// build.Default or a specified *build.Context.
 	var wordSize int64 = 8
-	switch impctx.Build.GOARCH {
+	switch conf.Build.GOARCH {
 	case "386", "arm":
 		wordSize = 4
 	}
-	impctx.TypeChecker.Sizes = &types.StdSizes{
+	conf.TypeChecker.Sizes = &types.StdSizes{
 		MaxAlign: 8,
 		WordSize: wordSize,
 	}
 
-	var debugMode bool
 	var mode ssa.BuilderMode
 	for _, c := range *buildFlag {
 		switch c {
 		case 'D':
-			debugMode = true
+			mode |= ssa.GlobalDebug
 		case 'P':
 			mode |= ssa.LogPackages | ssa.BuildSerially
 		case 'F':
@@ -106,7 +105,7 @@ func main() {
 		case 'N':
 			mode |= ssa.NaiveForm
 		case 'G':
-			impctx.SourceImports = false
+			conf.SourceImports = false
 		case 'L':
 			mode |= ssa.BuildSerially
 		default:
@@ -141,59 +140,52 @@ func main() {
 		defer pprof.StopCPUProfile()
 	}
 
-	// Load, parse and type-check the program.
-	imp := importer.New(&impctx)
-	infos, args, err := imp.LoadInitialPackages(args)
+	// Use the initial packages from the command line.
+	args, err := conf.FromArgs(args)
 	if err != nil {
 		log.Fatal(err)
 	}
 
 	// The interpreter needs the runtime package.
 	if *runFlag {
-		if _, err := imp.ImportPackage("runtime"); err != nil {
-			log.Fatalf("ImportPackage(runtime) failed: %s", err)
-		}
+		conf.Import("runtime")
 	}
 
-	// Create and build SSA-form program representation.
-	prog := ssa.NewProgram(imp.Fset, mode)
-	if err := prog.CreatePackages(imp); err != nil {
+	// Load, parse and type-check the whole program.
+	iprog, err := conf.Load()
+	if err != nil {
 		log.Fatal(err)
 	}
 
-	if debugMode {
-		for _, pkg := range prog.AllPackages() {
-			pkg.SetDebugMode(true)
-		}
-	}
+	// Create and build SSA-form program representation.
+	prog := ssa.Create(iprog, mode)
 	prog.BuildAll()
 
 	// Run the interpreter.
 	if *runFlag {
-		// If some package defines main, run that.
-		// Otherwise run all package's tests.
+		// If a package named "main" defines func main, run that.
+		// Otherwise run all packages' tests.
 		var main *ssa.Package
-		var pkgs []*ssa.Package
-		for _, info := range infos {
-			pkg := prog.Package(info.Pkg)
-			if pkg.Func("main") != nil {
+		pkgs := prog.AllPackages()
+		for _, pkg := range pkgs {
+			if pkg.Object.Name() == "main" && pkg.Func("main") != nil {
 				main = pkg
 				break
 			}
-			pkgs = append(pkgs, pkg)
 		}
-		if main == nil && pkgs != nil {
+		if main == nil && len(pkgs) > 0 {
+			// TODO(adonovan): only run tests if -test flag specified.
 			main = prog.CreateTestMainPackage(pkgs...)
 		}
 		if main == nil {
 			log.Fatal("No main package and no tests")
 		}
 
-		if runtime.GOARCH != impctx.Build.GOARCH {
+		if runtime.GOARCH != conf.Build.GOARCH {
 			log.Fatalf("Cross-interpretation is not yet supported (target has GOARCH %s, interpreter has %s).",
-				impctx.Build.GOARCH, runtime.GOARCH)
+				conf.Build.GOARCH, runtime.GOARCH)
 		}
 
-		interp.Interpret(main, interpMode, impctx.TypeChecker.Sizes, main.Object.Path(), args)
+		interp.Interpret(main, interpMode, conf.TypeChecker.Sizes, main.Object.Path(), args)
 	}
 }
diff --git a/importer/importer.go b/importer/importer.go
index bff5051a..0a86cc3a 100644
--- a/importer/importer.go
+++ b/importer/importer.go
@@ -2,9 +2,47 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Package importer defines the Importer, which loads, parses and
-// type-checks packages of Go code plus their transitive closure, and
-// retains both the ASTs and the derived facts.
+// Package importer loads, parses and type-checks packages of Go code
+// plus their transitive closure, and retains both the ASTs and the
+// derived facts.
+//
+// THIS INTERFACE IS EXPERIMENTAL AND IS LIKELY TO CHANGE.
+//
+// The package defines two primary types: Config, which specifies a
+// set of initial packages to load and various other options; and
+// Program, which is the result of successfully loading the packages
+// specified by a configuration.
+//
+// The configuration can be set directly, but *Config provides various
+// convenience methods to simplify the common cases, each of which can
+// be called any number of times.  Finally, these are followed by a
+// call to Load() to actually load and type-check the program.
+//
+//      var conf importer.Config
+//
+//      // Use the command-line arguments to specify
+//      // a set of initial packages to load from source.
+//      // See FromArgsUsage for help.
+//      rest, err := conf.FromArgs(os.Args[1:])
+//
+//      // Parse the specified files and create an ad-hoc package.
+//      // All files must have the same 'package' declaration.
+//      err := conf.CreateFromFilenames("foo.go", "bar.go")
+//
+//      // Create an ad-hoc package from the specified already-parsed files.
+//      // All ASTs must have the same 'package' declaration.
+//      err := conf.CreateFromFiles(parsedFiles)
+//
+//      // Add "runtime" to the set of packages to be loaded.
+//      err := conf.Import("runtime")
+//
+//      // Adds "fmt" and "fmt_test" to the set of packages
+//      // to be loaded.  "fmt" will include *_test.go files.
+//      err := conf.ImportWithTests("fmt")
+//
+//      // Finally, load all the packages specified by the configuration.
+//      prog, err := conf.Load()
+//
 //
 // CONCEPTS AND TERMINOLOGY
 //
@@ -17,11 +55,14 @@
 // same directory.  (go/build.Package calls these files XTestFiles.)
 //
 // An IMPORTABLE package is one that can be referred to by some import
-// spec.  Ad-hoc packages and external test packages are non-importable.
-// The importer and its clients must be careful not to assume that
-// the import path of a package may be used for a name-based lookup.
-// For example, a pointer analysis scope may consist of two initial
-// (ad-hoc) packages both called "main".
+// spec.  The Path() of each importable package is unique within a
+// Program.
+//
+// Ad-hoc packages and external test packages are NON-IMPORTABLE.  The
+// Path() of an ad-hoc package is inferred from the package
+// declarations of its files and is therefore not a unique package key.
+// For example, Config.CreatePkgs may specify two initial ad-hoc
+// packages both called "main".
 //
 // An AUGMENTED package is an importable package P plus all the
 // *_test.go files with same 'package foo' declaration as P.
@@ -40,13 +81,25 @@
 // Importer will only augment (and create an external test package
 // for) the first import path specified on the command-line.
 //
+// The INITIAL packages are those specified in the configuration.  A
+// DEPENDENCY is a package loaded to satisfy an import in an initial
+// package or another dependency.
+//
 package importer
 
+// TODO(adonovan):
+// - Rename this package go.tools/go/loader.
+// - (*Config).ParseFile is very handy, but feels like feature creep.
+//   (*Config).CreateFromFiles has a nasty precondition.
+// - Ideally some of this logic would move under the umbrella of
+//   go/types; see bug 7114.
+
 import (
 	"errors"
 	"fmt"
 	"go/ast"
 	"go/build"
+	"go/parser"
 	"go/token"
 	"os"
 	"strings"
@@ -57,30 +110,20 @@ import (
 	"code.google.com/p/go.tools/go/types"
 )
 
-// An Importer's exported methods are not thread-safe.
-type Importer struct {
-	Fset        *token.FileSet         // position info for all files seen
-	Config      *Config                // the client configuration, unmodified
-	importfn    types.Importer         // client's type import function
-	srcpkgs     map[string]bool        // for each package to load from source, whether to augment
-	allPackages []*PackageInfo         // all packages, including non-importable ones
-	imported    map[string]*importInfo // all imported packages (incl. failures) by import path
-}
-
-// importInfo holds internal information about each import path.
-type importInfo struct {
-	path string       // import path
-	info *PackageInfo // results of typechecking (including type errors)
-	err  error        // reason for failure to construct a package
-}
-
-// Config specifies the configuration for the importer.
+// Config specifies the configuration for a program to load.
 // The zero value for Config is a ready-to-use default configuration.
 type Config struct {
+	// Fset is the file set for the parser to use when loading the
+	// program.  If nil, it will be lazily initialized by any
+	// method of Config.
+	Fset *token.FileSet
+
 	// TypeChecker contains options relating to the type checker.
 	//
 	// The supplied IgnoreFuncBodies is not used; the effective
 	// value comes from the TypeCheckFuncBodies func below.
+	//
+	// TypeChecker.Packages is lazily initialized during Load.
 	TypeChecker types.Config
 
 	// TypeCheckFuncBodies is a predicate over package import
@@ -91,209 +134,91 @@ type Config struct {
 	// checked.
 	TypeCheckFuncBodies func(string) bool
 
-	// SourceImports determines whether to satisfy all imports by
+	// SourceImports determines whether to satisfy dependencies by
 	// loading Go source code.
 	//
+	// If true, the entire program---the initial packages and
+	// their transitive closure of dependencies---will be loaded,
+	// parsed and type-checked.  This is required for
+	// whole-program analyses such as pointer analysis.
+	//
 	// If false, the TypeChecker.Import mechanism will be used
 	// instead.  Since that typically supplies only the types of
 	// package-level declarations and values of constants, but no
 	// code, it will not yield a whole program.  It is intended
 	// for analyses that perform intraprocedural analysis of a
-	// single package.
+	// single package, e.g. traditional compilation.
 	//
-	// The importer's initial packages are always loaded from
-	// source, regardless of this flag's setting.
+	// The initial packages (CreatePkgs and ImportPkgs) are always
+	// loaded from Go source, regardless of this flag's setting.
 	SourceImports bool
 
 	// If Build is non-nil, it is used to locate source packages.
 	// Otherwise &build.Default is used.
 	Build *build.Context
+
+	// CreatePkgs specifies a list of non-importable initial
+	// packages to create.  Each element is a list of parsed files
+	// to be type-checked into a new package whose name is taken
+	// from ast.File.Package.
+	//
+	// The resulting packages will appear in the corresponding
+	// elements of the Program.Created slice.
+	CreatePkgs [][]*ast.File
+
+	// ImportPkgs specifies a set of initial packages to load from
+	// source.  The map keys are package import paths, used to
+	// locate the package relative to $GOROOT.  The corresponding
+	// values indicate whether to augment the package by *_test.go
+	// files.
+	//
+	// Due to current type-checker limitations, at most one entry
+	// may be augmented (true).
+	ImportPkgs map[string]bool
 }
 
-// build returns the effective build context.
-func (c *Config) build() *build.Context {
-	if c.Build != nil {
-		return c.Build
-	}
-	return &build.Default
+// A Program is a Go program loaded from source or binary
+// as specified by a Config.
+type Program struct {
+	Fset *token.FileSet // the file set for this program
+
+	// Created[i] contains the initial package whose ASTs were
+	// supplied by Config.CreatePkgs[i].
+	Created []*PackageInfo
+
+	// Imported contains the initially imported packages,
+	// as specified by Config.ImportPkgs.
+	Imported map[string]*PackageInfo
+
+	// ImportMap is the canonical mapping of import paths to
+	// packages used by the type-checker (Config.TypeChecker.Packages).
+	// It contains all Imported initial packages, but not Created
+	// ones, and all imported dependencies.
+	ImportMap map[string]*types.Package
+
+	// AllPackages contains the PackageInfo of every package
+	// encountered by Load: all initial packages and all
+	// dependencies, including incomplete ones.
+	AllPackages map[*types.Package]*PackageInfo
 }
 
-// New returns a new, empty Importer using configuration options
-// specified by config.
-//
-func New(config *Config) *Importer {
-	// Initialize by mutating the caller's copy,
-	// so all copies agree on the identity of the map.
-	if config.TypeChecker.Packages == nil {
-		config.TypeChecker.Packages = make(map[string]*types.Package)
+func (conf *Config) fset() *token.FileSet {
+	if conf.Fset == nil {
+		conf.Fset = token.NewFileSet()
 	}
-
-	// Save the caller's effective Import funcion.
-	importfn := config.TypeChecker.Import
-	if importfn == nil {
-		importfn = gcimporter.Import
-	}
-
-	imp := &Importer{
-		Fset:     token.NewFileSet(),
-		Config:   config,
-		importfn: importfn,
-		srcpkgs:  make(map[string]bool),
-		imported: make(map[string]*importInfo),
-	}
-	return imp
+	return conf.Fset
 }
 
-// AllPackages returns a new slice containing all complete packages
-// loaded by importer imp.
+// ParseFile is a convenience function that invokes the parser using
+// the Config's FileSet, which is initialized if nil.
 //
-// This returns only packages that were loaded from source or directly
-// imported from a source package.  It does not include packages
-// indirectly referenced by a binary package; they are found in
-// config.TypeChecker.Packages.
-// TODO(adonovan): rethink this API.
-//
-func (imp *Importer) AllPackages() []*PackageInfo {
-	return append([]*PackageInfo(nil), imp.allPackages...)
+func (conf *Config) ParseFile(filename string, src interface{}, mode parser.Mode) (*ast.File, error) {
+	return parser.ParseFile(conf.fset(), filename, src, mode)
 }
 
-// doImport imports the package denoted by path.
-// It implements the types.Importer prototype.
-//
-// imports is the import map of the importing package, later
-// accessible as types.Package.Imports().  If non-nil, doImport will
-// update it to include this import.  (It may be nil in recursive
-// calls for prefetching.)
-//
-// It returns an error if a package could not be created
-// (e.g. go/build or parse error), but type errors are reported via
-// the types.Config.Error callback (the first of which is also saved
-// in the package's PackageInfo).
-//
-// Idempotent.
-//
-func (imp *Importer) doImport(imports map[string]*types.Package, path string) (*types.Package, error) {
-	// Package unsafe is handled specially, and has no PackageInfo.
-	// TODO(adonovan): a fake empty package would make things simpler.
-	if path == "unsafe" {
-		return types.Unsafe, nil
-	}
-
-	info, err := imp.ImportPackage(path)
-	if err != nil {
-		return nil, err
-	}
-
-	// Update the type checker's package map on success.
-	imports[path] = info.Pkg
-
-	return info.Pkg, nil
-}
-
-// ImportPackage imports the package whose import path is path, plus
-// its necessary dependencies.
-//
-// Precondition: path != "unsafe".
-//
-func (imp *Importer) ImportPackage(path string) (*PackageInfo, error) {
-	ii, ok := imp.imported[path]
-	if !ok {
-		ii = &importInfo{path: path}
-		imp.imported[path] = ii
-
-		// Find and create the actual package.
-		if augment, ok := imp.srcpkgs[ii.path]; ok || imp.Config.SourceImports {
-			which := "g" // load *.go files
-			if augment {
-				which = "gt" // augment package by in-package *_test.go files
-			}
-			imp.loadFromSource(ii, which)
-		} else {
-			imp.loadFromBinary(ii)
-		}
-		if ii.info != nil {
-			ii.info.Importable = true
-		}
-	}
-
-	return ii.info, ii.err
-}
-
-// loadFromBinary implements package loading from the client-supplied
-// external source, e.g. object files from the gc compiler.
-//
-func (imp *Importer) loadFromBinary(ii *importInfo) {
-	pkg, err := imp.importfn(imp.Config.TypeChecker.Packages, ii.path)
-	if pkg != nil {
-		ii.info = &PackageInfo{Pkg: pkg}
-		imp.allPackages = append(imp.allPackages, ii.info)
-	} else {
-		ii.err = err
-	}
-}
-
-// loadFromSource implements package loading by parsing Go source files
-// located by go/build.  which indicates which files to include in the
-// package.
-//
-func (imp *Importer) loadFromSource(ii *importInfo, which string) {
-	if files, err := parsePackageFiles(imp.Config.build(), imp.Fset, ii.path, which); err == nil {
-		// Type-check the package.
-		ii.info = imp.CreatePackage(ii.path, files...)
-	} else {
-		ii.err = err
-	}
-}
-
-// CreatePackage creates and type-checks a package from the specified
-// list of parsed files, importing their dependencies.  It returns a
-// PackageInfo containing the resulting types.Package, the ASTs, and
-// other type information.
-//
-// The order of files determines the package initialization order.
-//
-// path is the full name under which this package is known, such as
-// appears in an import declaration. e.g. "sync/atomic".  It need not
-// be unique; for example, it is possible to construct two distinct
-// packages both named "main".
-//
-// The resulting package is accessible via AllPackages() but is not
-// importable, i.e. no 'import' spec can resolve to it.
-//
-// CreatePackage never fails, but the resulting package may contain type
-// errors; the first of these is recorded in PackageInfo.Err.
-//
-func (imp *Importer) CreatePackage(path string, files ...*ast.File) *PackageInfo {
-	info := &PackageInfo{
-		Files: files,
-		Info: types.Info{
-			Types:      make(map[ast.Expr]types.Type),
-			Values:     make(map[ast.Expr]exact.Value),
-			Objects:    make(map[*ast.Ident]types.Object),
-			Implicits:  make(map[ast.Node]types.Object),
-			Scopes:     make(map[ast.Node]*types.Scope),
-			Selections: make(map[*ast.SelectorExpr]*types.Selection),
-		},
-	}
-
-	// Use a copy of the types.Config so we can vary IgnoreFuncBodies.
-	tc := imp.Config.TypeChecker
-	tc.IgnoreFuncBodies = false
-	if f := imp.Config.TypeCheckFuncBodies; f != nil {
-		tc.IgnoreFuncBodies = !f(path)
-	}
-	if tc.Error == nil {
-		tc.Error = func(e error) { fmt.Fprintln(os.Stderr, e) }
-	}
-	tc.Import = imp.doImport // doImport wraps the user's importfn, effectively
-	info.Pkg, info.Err = tc.Check(path, imp.Fset, files, &info.Info)
-	imp.allPackages = append(imp.allPackages, info)
-	return info
-}
-
-// InitialPackagesUsage is a partial usage message that client
-// applications may wish to include in their -help output.
-const InitialPackagesUsage = `
+// FromArgsUsage is a partial usage message that applications calling
+// FromArgs may wish to include in their -help output.
+const FromArgsUsage = `
 <args> is a list of arguments denoting a set of initial packages.
 Each argument may take one of two forms:
 
@@ -326,41 +251,15 @@ Each argument may take one of two forms:
 A '--' argument terminates the list of packages.
 `
 
-// LoadInitialPackages interprets args as a set of packages, loads
-// those packages and their dependencies, and returns them.
+// FromArgs interprets args as a set of initial packages to load from
+// source and updates the configuration.  It returns the list of
+// unconsumed arguments.
 //
 // It is intended for use in command-line interfaces that require a
-// set of initial packages to be specified; see InitialPackagesUsage
-// message for details.
+// set of initial packages to be specified; see FromArgsUsage message
+// for details.
 //
-// The second result parameter returns the list of unconsumed
-// arguments.
-//
-// It is an error to specify no packages.
-//
-// Precondition: LoadInitialPackages cannot be called after any
-// previous calls to Load* on the same importer.
-//
-func (imp *Importer) LoadInitialPackages(args []string) ([]*PackageInfo, []string, error) {
-	// The "augmentation" mechanism requires that we mark all
-	// packages to be augmented before we import a single one.
-	if len(imp.allPackages) > 0 {
-		return nil, nil, errors.New("LoadInitialPackages called on non-pristine Importer")
-	}
-
-	// We use two passes.  The first parses the files for each
-	// non-importable package and discovers the set of importable
-	// packages that require augmentation by in-package _test.go
-	// files.  The second creates the ad-hoc packages and imports
-	// the importable ones.
-	//
-	// This is necessary to ensure that all packages requiring
-	// augmentation are known before before any package is
-	// imported.
-
-	// Pass 1: parse the sets of files for each package.
-	var pkgs []*initialPkg
-	var seenAugmented bool
+func (conf *Config) FromArgs(args []string) (rest []string, err error) {
 	for len(args) > 0 {
 		arg := args[0]
 		args = args[1:]
@@ -371,134 +270,114 @@ func (imp *Importer) LoadInitialPackages(args []string) ([]*PackageInfo, []strin
 		if strings.HasSuffix(arg, ".go") {
 			// Assume arg is a comma-separated list of *.go files
 			// comprising a single package.
-			pkg, err := initialPackageFromFiles(imp.Fset, arg)
-			if err != nil {
-				return nil, nil, err
-			}
-			pkgs = append(pkgs, pkg)
-
+			err = conf.CreateFromFilenames(strings.Split(arg, ",")...)
 		} else {
 			// Assume arg is a directory name denoting a
 			// package, perhaps plus an external test
 			// package unless prefixed by "notest:".
-			path := strings.TrimPrefix(arg, "notest:")
-
-			if path == "unsafe" {
-				continue // ignore; has no PackageInfo
+			if path := strings.TrimPrefix(arg, "notest:"); path != arg {
+				conf.Import(path)
+			} else {
+				err = conf.ImportWithTests(path)
 			}
-
-			pkgs = append(pkgs, &initialPkg{
-				path:       path,
-				importable: true,
-			})
-			imp.srcpkgs[path] = false // unaugmented source package
-
-			if path != arg {
-				continue // had "notest:" prefix
-			}
-
-			// TODO(adonovan): due to limitations of the current type
-			// checker design, we can augment at most one package.
-			if seenAugmented {
-				continue // don't attempt a second
-			}
-
-			// Load the external test package.
-			xtestFiles, err := parsePackageFiles(imp.Config.build(), imp.Fset, path, "x")
-			if err != nil {
-				return nil, nil, err
-			}
-			if len(xtestFiles) > 0 {
-				pkgs = append(pkgs, &initialPkg{
-					path:       path + "_test",
-					importable: false,
-					files:      xtestFiles,
-				})
-			}
-
-			// Mark the non-xtest package for augmentation with
-			// in-package *_test.go files when we import it below.
-			imp.srcpkgs[path] = true
-			seenAugmented = true
 		}
-	}
-
-	// Pass 2: type-check each set of files to make a package.
-	var infos []*PackageInfo
-	for _, pkg := range pkgs {
-		var info *PackageInfo
-		if pkg.importable {
-			var err error
-			info, err = imp.ImportPackage(pkg.path)
-			if err != nil {
-				return nil, nil, err // e.g. parse error (but not type error)
-			}
-		} else {
-			info = imp.CreatePackage(pkg.path, pkg.files...)
-		}
-		infos = append(infos, info)
-	}
-
-	if len(pkgs) == 0 {
-		return nil, nil, errors.New("no *.go source files nor packages were specified")
-	}
-
-	return infos, args, nil
-}
-
-type initialPkg struct {
-	path       string      // the package's import path
-	importable bool        // add package to import map (false for main and xtests)
-	files      []*ast.File // set of files (non-importable packages only)
-}
-
-// initialPackageFromFiles returns an initialPkg, given a
-// comma-separated list of *.go source files belonging to the same
-// directory and possessing the same 'package decl'.
-//
-func initialPackageFromFiles(fset *token.FileSet, arg string) (*initialPkg, error) {
-	filenames := strings.Split(arg, ",")
-	for _, filename := range filenames {
-		if !strings.HasSuffix(filename, ".go") {
-			return nil, fmt.Errorf("not a *.go source file: %q", filename)
-		}
-	}
-
-	files, err := ParseFiles(fset, ".", filenames...)
-	if err != nil {
-		return nil, err
-	}
-
-	// Take the package name from the 'package decl' in each file,
-	// all of which must match.
-	pkgname := files[0].Name.Name
-	for i, file := range files[1:] {
-		if pn := file.Name.Name; pn != pkgname {
-			err := fmt.Errorf("can't load package: found packages %s (%s) and %s (%s)",
-				pkgname, filenames[0], pn, filenames[i])
+		if err != nil {
 			return nil, err
 		}
-		// TODO(adonovan): check dirnames are equal, like 'go build' does.
+	}
+	return args, nil
+}
+
+// CreateFromFilenames is a convenience function that parses the
+// specified *.go files and adds a package entry for them to
+// conf.CreatePkgs.
+//
+func (conf *Config) CreateFromFilenames(filenames ...string) error {
+	files, err := parseFiles(conf.fset(), ".", filenames...)
+	if err != nil {
+		return err
 	}
 
-	return &initialPkg{
-		path:       pkgname,
-		importable: false,
-		files:      files,
-	}, nil
+	conf.CreateFromFiles(files...)
+	return nil
+}
+
+// CreateFromFiles is a convenience function that adds a CreatePkgs
+// entry for the specified parsed files.
+//
+// Precondition: conf.Fset is non-nil and was the fileset used to parse
+// the files.  (e.g. the files came from conf.ParseFile().)
+//
+func (conf *Config) CreateFromFiles(files ...*ast.File) {
+	if conf.Fset == nil {
+		panic("nil Fset")
+	}
+	conf.CreatePkgs = append(conf.CreatePkgs, files)
+}
+
+// ImportWithTests is a convenience function that adds path to
+// ImportPkgs, the set of initial source packages located relative to
+// $GOPATH.  The package will be augmented by any *_test.go files in
+// its directory that contain a "package x" (not "package x_test")
+// declaration.
+//
+// In addition, if any *_test.go files contain a "package <path>_test"
+// declaration, an additional package comprising just those files will
+// be added to CreatePkgs.
+//
+func (conf *Config) ImportWithTests(path string) error {
+	if path == "unsafe" {
+		return nil // ignore; not a real package
+	}
+	conf.Import(path)
+
+	// TODO(adonovan): due to limitations of the current type
+	// checker design, we can augment at most one package.
+	for _, augmented := range conf.ImportPkgs {
+		if augmented {
+			return nil // don't attempt a second
+		}
+	}
+
+	// Load the external test package.
+	xtestFiles, err := parsePackageFiles(conf.build(), conf.fset(), path, "x")
+	if err != nil {
+		return err
+	}
+	if len(xtestFiles) > 0 {
+		conf.CreateFromFiles(xtestFiles...)
+	}
+
+	// Mark the non-xtest package for augmentation with
+	// in-package *_test.go files when we import it below.
+	conf.ImportPkgs[path] = true
+	return nil
+}
+
+// Import is a convenience function that adds path to ImportPkgs, the
+// set of initial packages that will be imported from source.
+//
+func (conf *Config) Import(path string) {
+	if path == "unsafe" {
+		return // ignore; not a real package
+	}
+	if conf.ImportPkgs == nil {
+		conf.ImportPkgs = make(map[string]bool)
+	}
+	conf.ImportPkgs[path] = false // unaugmented source package
 }
 
 // PathEnclosingInterval returns the PackageInfo and ast.Node that
 // contain source interval [start, end), and all the node's ancestors
-// up to the AST root.  It searches all ast.Files of all packages in the
-// Importer imp.  exact is defined as for astutil.PathEnclosingInterval.
+// up to the AST root.  It searches all ast.Files of all packages in prog.
+// exact is defined as for astutil.PathEnclosingInterval.
 //
 // The result is (nil, nil, false) if not found.
 //
-func (imp *Importer) PathEnclosingInterval(start, end token.Pos) (pkg *PackageInfo, path []ast.Node, exact bool) {
-	for _, info := range imp.allPackages {
+func (prog *Program) PathEnclosingInterval(start, end token.Pos) (pkg *PackageInfo, path []ast.Node, exact bool) {
+	for _, info := range prog.AllPackages {
 		for _, f := range info.Files {
-			if !tokenFileContainsPos(imp.Fset.File(f.Package), start) {
+			if !tokenFileContainsPos(prog.Fset.File(f.Pos()), start) {
 				continue
 			}
 			if path, exact := astutil.PathEnclosingInterval(f, start, end); path != nil {
@@ -509,9 +388,252 @@ func (imp *Importer) PathEnclosingInterval(start, end token.Pos) (pkg *PackageIn
 	return nil, nil, false
 }
 
-// TODO(adonovan): make this a method: func (*token.File) Contains(token.Pos)
-func tokenFileContainsPos(f *token.File, pos token.Pos) bool {
-	p := int(pos)
-	base := f.Base()
-	return base <= p && p < base+f.Size()
+// InitialPackages returns a new slice containing the set of initial
+// packages (Created + Imported) in unspecified order.
+//
+func (prog *Program) InitialPackages() []*PackageInfo {
+	infos := make([]*PackageInfo, 0, len(prog.Created)+len(prog.Imported))
+	infos = append(infos, prog.Created...)
+	for _, info := range prog.Imported {
+		infos = append(infos, info)
+	}
+	return infos
+}
+
+// ---------- Implementation ----------
+
+// importer holds the working state of the algorithm.
+type importer struct {
+	conf     *Config                // the client configuration
+	prog     *Program               // resulting program
+	imported map[string]*importInfo // all imported packages (incl. failures) by import path
+}
+
+// importInfo tracks the success or failure of a single import.
+type importInfo struct {
+	info *PackageInfo // results of typechecking (including type errors)
+	err  error        // reason for failure to make a package
+}
+
+// Load creates the initial packages specified by conf.{Create,Import}Pkgs,
+// loading their dependencies packages as needed.
+//
+// On success, it returns a Program containing a PackageInfo for each
+// package; all are well-typed.
+//
+// It is an error if no packages were loaded.
+//
+func (conf *Config) Load() (*Program, error) {
+	// Initialize by setting the conf's copy, so all copies of
+	// TypeChecker agree on the identity of the map.
+	if conf.TypeChecker.Packages == nil {
+		conf.TypeChecker.Packages = make(map[string]*types.Package)
+	}
+
+	prog := &Program{
+		Fset:        conf.fset(),
+		Imported:    make(map[string]*PackageInfo),
+		ImportMap:   conf.TypeChecker.Packages,
+		AllPackages: make(map[*types.Package]*PackageInfo),
+	}
+
+	imp := importer{
+		conf:     conf,
+		prog:     prog,
+		imported: make(map[string]*importInfo),
+	}
+
+	for path := range conf.ImportPkgs {
+		info, err := imp.importPackage(path)
+		if err != nil {
+			return nil, err // e.g. parse error (but not type error)
+		}
+		prog.Imported[path] = info
+	}
+
+	for _, files := range conf.CreatePkgs {
+		pkgname, err := packageName(files, conf.Fset)
+		if err != nil {
+			return nil, err
+		}
+		// TODO(adonovan): pkgnames are not unique, but the
+		// typechecker assumes they are in its Id() logic.
+		prog.Created = append(prog.Created, imp.createPackage(pkgname, files...))
+	}
+
+	if len(prog.Imported)+len(prog.Created) == 0 {
+		return nil, errors.New("no *.go source files nor packages were specified")
+	}
+
+	// Report errors in indirectly imported packages.
+	var errpkgs []string
+	for _, info := range prog.AllPackages {
+		if info.err != nil {
+			errpkgs = append(errpkgs, info.Pkg.Path())
+		}
+	}
+	if errpkgs != nil {
+		return nil, fmt.Errorf("couldn't load packages due to type errors: %s",
+			strings.Join(errpkgs, ", "))
+	}
+
+	// Create infos for indirectly imported packages.
+	// e.g. incomplete packages without syntax, loaded from export data.
+	for _, obj := range prog.ImportMap {
+		if prog.AllPackages[obj] == nil {
+			prog.AllPackages[obj] = &PackageInfo{Pkg: obj, Importable: true}
+		}
+	}
+
+	return prog, nil
+}
+
+// build returns the effective build context.
+func (conf *Config) build() *build.Context {
+	if conf.Build != nil {
+		return conf.Build
+	}
+	return &build.Default
+}
+
+// doImport imports the package denoted by path.
+// It implements the types.Importer signature.
+//
+// imports is the import map of the importing package, later
+// accessible as types.Package.Imports().  If non-nil, doImport will
+// update it to include this import.  (It may be nil in recursive
+// calls for prefetching.)
+//
+// It returns an error if a package could not be created
+// (e.g. go/build or parse error), but type errors are reported via
+// the types.Config.Error callback (the first of which is also saved
+// in the package's PackageInfo).
+//
+// Idempotent.
+//
+func (imp *importer) doImport(imports map[string]*types.Package, path string) (*types.Package, error) {
+	// Package unsafe is handled specially, and has no PackageInfo.
+	if path == "unsafe" {
+		return types.Unsafe, nil
+	}
+
+	info, err := imp.importPackage(path)
+	if err != nil {
+		return nil, err
+	}
+
+	// Update the type checker's package map on success.
+	imports[path] = info.Pkg
+
+	return info.Pkg, nil
+}
+
+// importPackage imports the package with the given import path, plus
+// its dependencies.
+//
+// Precondition: path != "unsafe".
+//
+func (imp *importer) importPackage(path string) (*PackageInfo, error) {
+	ii, ok := imp.imported[path]
+	if !ok {
+		// In preorder, initialize the map entry to a cycle
+		// error in case importPackage(path) is called again
+		// before the import is completed.
+		// TODO(adonovan): go/types should be responsible for
+		// reporting cycles; see bug 7114.
+		ii = &importInfo{err: fmt.Errorf("import cycle in package %s", path)}
+		imp.imported[path] = ii
+
+		// Find and create the actual package.
+		if augment, ok := imp.conf.ImportPkgs[path]; ok || imp.conf.SourceImports {
+			which := "g" // load *.go files
+			if augment {
+				which = "gt" // augment package by in-package *_test.go files
+			}
+
+			ii.info, ii.err = imp.importFromSource(path, which)
+		} else {
+			ii.info, ii.err = imp.importFromBinary(path)
+		}
+		if ii.info != nil {
+			ii.info.Importable = true
+		}
+	}
+
+	return ii.info, ii.err
+}
+
+// importFromBinary implements package loading from the client-supplied
+// external source, e.g. object files from the gc compiler.
+//
+func (imp *importer) importFromBinary(path string) (*PackageInfo, error) {
+	// Determine the caller's effective Import function.
+	importfn := imp.conf.TypeChecker.Import
+	if importfn == nil {
+		importfn = gcimporter.Import
+	}
+	pkg, err := importfn(imp.conf.TypeChecker.Packages, path)
+	if err != nil {
+		return nil, err
+	}
+	info := &PackageInfo{Pkg: pkg}
+	imp.prog.AllPackages[pkg] = info
+	return info, nil
+}
+
+// importFromSource implements package loading by parsing Go source files
+// located by go/build.  which indicates which files to include in the
+// package.
+//
+func (imp *importer) importFromSource(path string, which string) (*PackageInfo, error) {
+	files, err := parsePackageFiles(imp.conf.build(), imp.conf.fset(), path, which)
+	if err != nil {
+		return nil, err
+	}
+	// Type-check the package.
+	return imp.createPackage(path, files...), nil
+}
+
+// createPackage creates and type-checks a package from the specified
+// list of parsed files, importing their dependencies.  It returns a
+// PackageInfo containing the resulting types.Package, the ASTs, and
+// other type information.
+//
+// The order of files determines the package initialization order.
+//
+// path will be the resulting package's Path().
+// For an ad-hoc package, this is not necessarily unique.
+//
+// The resulting package is accessible via AllPackages but is not
+// importable, i.e. no 'import' spec can resolve to it.
+//
+// createPackage never fails, but the resulting package may contain type
+// errors; the first of these is recorded in PackageInfo.err.
+//
+func (imp *importer) createPackage(path string, files ...*ast.File) *PackageInfo {
+	info := &PackageInfo{
+		Files: files,
+		Info: types.Info{
+			Types:      make(map[ast.Expr]types.Type),
+			Values:     make(map[ast.Expr]exact.Value),
+			Objects:    make(map[*ast.Ident]types.Object),
+			Implicits:  make(map[ast.Node]types.Object),
+			Scopes:     make(map[ast.Node]*types.Scope),
+			Selections: make(map[*ast.SelectorExpr]*types.Selection),
+		},
+	}
+
+	// Use a copy of the types.Config so we can vary IgnoreFuncBodies.
+	tc := imp.conf.TypeChecker
+	tc.IgnoreFuncBodies = false
+	if f := imp.conf.TypeCheckFuncBodies; f != nil {
+		tc.IgnoreFuncBodies = !f(path)
+	}
+	if tc.Error == nil {
+		tc.Error = func(e error) { fmt.Fprintln(os.Stderr, e) }
+	}
+	tc.Import = imp.doImport // doImport wraps the user's importfn, effectively
+	info.Pkg, info.err = tc.Check(path, imp.conf.fset(), files, &info.Info)
+	imp.prog.AllPackages[info.Pkg] = info
+	return info
 }
diff --git a/importer/importer_test.go b/importer/importer_test.go
index 0c868b55..e995df26 100644
--- a/importer/importer_test.go
+++ b/importer/importer_test.go
@@ -6,52 +6,73 @@ package importer_test
 
 import (
 	"fmt"
+	"sort"
 	"testing"
 
 	"code.google.com/p/go.tools/importer"
 )
 
-func TestLoadInitialPackages(t *testing.T) {
+func loadFromArgs(args []string) (prog *importer.Program, rest []string, err error) {
+	conf := &importer.Config{}
+	rest, err = conf.FromArgs(args)
+	if err == nil {
+		prog, err = conf.Load()
+	}
+	return
+}
+
+func TestLoadFromArgs(t *testing.T) {
 	// Failed load: bad first import path causes parsePackageFiles to fail.
 	args := []string{"nosuchpkg", "errors"}
-	if _, _, err := importer.New(&importer.Config{}).LoadInitialPackages(args); err == nil {
-		t.Errorf("LoadInitialPackages(%q) succeeded, want failure", args)
+	if _, _, err := loadFromArgs(args); err == nil {
+		t.Errorf("loadFromArgs(%q) succeeded, want failure", args)
 	} else {
 		// cannot find package: ok.
 	}
 
 	// Failed load: bad second import path proceeds to doImport0, which fails.
 	args = []string{"errors", "nosuchpkg"}
-	if _, _, err := importer.New(&importer.Config{}).LoadInitialPackages(args); err == nil {
-		t.Errorf("LoadInitialPackages(%q) succeeded, want failure", args)
+	if _, _, err := loadFromArgs(args); err == nil {
+		t.Errorf("loadFromArgs(%q) succeeded, want failure", args)
 	} else {
 		// cannot find package: ok
 	}
 
 	// Successful load.
 	args = []string{"fmt", "errors", "testdata/a.go,testdata/b.go", "--", "surplus"}
-	imp := importer.New(&importer.Config{})
-	infos, rest, err := imp.LoadInitialPackages(args)
+	prog, rest, err := loadFromArgs(args)
 	if err != nil {
-		t.Errorf("LoadInitialPackages(%q) failed: %s", args, err)
+		t.Errorf("loadFromArgs(%q) failed: %s", args, err)
 		return
 	}
 	if got, want := fmt.Sprint(rest), "[surplus]"; got != want {
-		t.Errorf("LoadInitialPackages(%q) rest: got %s, want %s", got, want)
+		t.Errorf("loadFromArgs(%q) rest: got %s, want %s", got, want)
 	}
-	// Check list of initial packages.
+	// Check list of Created packages.
 	var pkgnames []string
-	for _, info := range infos {
+	for _, info := range prog.Created {
 		pkgnames = append(pkgnames, info.Pkg.Path())
 	}
 	// Only the first import path (currently) contributes tests.
-	if got, want := fmt.Sprint(pkgnames), "[fmt fmt_test errors P]"; got != want {
-		t.Errorf("InitialPackages: got %s, want %s", got, want)
+	if got, want := fmt.Sprint(pkgnames), "[fmt_test P]"; got != want {
+		t.Errorf("Created: got %s, want %s", got, want)
 	}
+
+	// Check set of Imported packages.
+	pkgnames = nil
+	for path := range prog.Imported {
+		pkgnames = append(pkgnames, path)
+	}
+	sort.Strings(pkgnames)
+	// Only the first import path (currently) contributes tests.
+	if got, want := fmt.Sprint(pkgnames), "[errors fmt]"; got != want {
+		t.Errorf("Loaded: got %s, want %s", got, want)
+	}
+
 	// Check set of transitive packages.
 	// There are >30 and the set may grow over time, so only check a few.
 	all := map[string]struct{}{}
-	for _, info := range imp.AllPackages() {
+	for _, info := range prog.AllPackages {
 		all[info.Pkg.Path()] = struct{}{}
 	}
 	want := []string{"strings", "time", "runtime", "testing", "unicode"}
diff --git a/importer/pkginfo.go b/importer/pkginfo.go
index 7cdfec1c..6432080a 100644
--- a/importer/pkginfo.go
+++ b/importer/pkginfo.go
@@ -20,8 +20,8 @@ import (
 type PackageInfo struct {
 	Pkg        *types.Package
 	Importable bool        // true if 'import "Pkg.Path()"' would resolve to this
-	Err        error       // non-nil if the package had static errors
 	Files      []*ast.File // abstract syntax for the package's files
+	err        error       // non-nil if the package had static errors
 	types.Info             // type-checker deductions.
 }
 
diff --git a/importer/source_test.go b/importer/source_test.go
index 84a1b4c0..bcb121c8 100644
--- a/importer/source_test.go
+++ b/importer/source_test.go
@@ -82,8 +82,8 @@ func TestEnclosingFunction(t *testing.T) {
 			"900", "func@2.27"},
 	}
 	for _, test := range tests {
-		imp := importer.New(&importer.Config{})
-		f, start, end := findInterval(t, imp.Fset, test.input, test.substr)
+		conf := importer.Config{Fset: token.NewFileSet()}
+		f, start, end := findInterval(t, conf.Fset, test.input, test.substr)
 		if f == nil {
 			continue
 		}
@@ -92,13 +92,16 @@ func TestEnclosingFunction(t *testing.T) {
 			t.Errorf("EnclosingFunction(%q) not exact", test.substr)
 			continue
 		}
-		mainInfo := imp.CreatePackage("main", f)
-		prog := ssa.NewProgram(imp.Fset, 0)
-		if err := prog.CreatePackages(imp); err != nil {
+
+		conf.CreateFromFiles(f)
+
+		iprog, err := conf.Load()
+		if err != nil {
 			t.Error(err)
 			continue
 		}
-		pkg := prog.Package(mainInfo.Pkg)
+		prog := ssa.Create(iprog, 0)
+		pkg := prog.Package(iprog.Created[0].Pkg)
 		pkg.Build()
 
 		name := "(none)"
diff --git a/importer/util.go b/importer/util.go
index 262c39f9..b824f573 100644
--- a/importer/util.go
+++ b/importer/util.go
@@ -8,12 +8,12 @@ package importer
 // and used by it.
 
 import (
+	"fmt"
 	"go/ast"
 	"go/build"
 	"go/parser"
 	"go/token"
 	"path/filepath"
-	"strconv"
 	"sync"
 )
 
@@ -55,13 +55,13 @@ func parsePackageFiles(ctxt *build.Context, fset *token.FileSet, path string, wh
 		}
 		filenames = append(filenames, s...)
 	}
-	return ParseFiles(fset, bp.Dir, filenames...)
+	return parseFiles(fset, bp.Dir, filenames...)
 }
 
-// ParseFiles parses the Go source files files within directory dir
+// parseFiles parses the Go source files files within directory dir
 // and returns their ASTs, or the first parse error if any.
 //
-func ParseFiles(fset *token.FileSet, dir string, files ...string) ([]*ast.File, error) {
+func parseFiles(fset *token.FileSet, dir string, files ...string) ([]*ast.File, error) {
 	var wg sync.WaitGroup
 	n := len(files)
 	parsed := make([]*ast.File, n, n)
@@ -104,26 +104,32 @@ func unreachable() {
 	panic("unreachable")
 }
 
-// importsOf returns the set of paths imported by the specified files.
-func importsOf(p string, files []*ast.File) map[string]bool {
-	imports := make(map[string]bool)
-outer:
-	for _, file := range files {
-		for _, decl := range file.Decls {
-			if decl, ok := decl.(*ast.GenDecl); ok {
-				if decl.Tok != token.IMPORT {
-					break outer // stop at the first non-import
-				}
-				for _, spec := range decl.Specs {
-					spec := spec.(*ast.ImportSpec)
-					if path, _ := strconv.Unquote(spec.Path.Value); path != "C" {
-						imports[path] = true
-					}
-				}
-			} else {
-				break outer // stop at the first non-import
-			}
-		}
+func packageName(files []*ast.File, fset *token.FileSet) (string, error) {
+	if len(files) == 0 {
+		return "", fmt.Errorf("no files in package")
 	}
-	return imports
+	// Take the package name from the 'package decl' in each file,
+	// all of which must match.
+	pkgname := files[0].Name.Name
+	for _, file := range files[1:] {
+		if pn := file.Name.Name; pn != pkgname {
+			err := fmt.Errorf("can't load package: found packages %s (%s) and %s (%s)",
+				pkgname, filename(files[0], fset),
+				pn, filename(file, fset))
+			return "", err
+		}
+		// TODO(adonovan): check dirnames are equal, like 'go build' does.
+	}
+	return pkgname, nil
+}
+
+// TODO(adonovan): make this a method: func (*token.File) Contains(token.Pos)
+func tokenFileContainsPos(f *token.File, pos token.Pos) bool {
+	p := int(pos)
+	base := f.Base()
+	return base <= p && p < base+f.Size()
+}
+
+func filename(file *ast.File, fset *token.FileSet) string {
+	return fset.File(file.Pos()).Name()
 }
diff --git a/oracle/oracle.go b/oracle/oracle.go
index c3596a0c..eeb7103b 100644
--- a/oracle/oracle.go
+++ b/oracle/oracle.go
@@ -183,7 +183,7 @@ func (res *Result) Serial() *serial.Result {
 
 // Query runs a single oracle query.
 //
-// args specify the main package in importer.LoadInitialPackages syntax.
+// args specify the main package in (*importer.Config).FromArgs syntax.
 // mode is the query mode ("callers", etc).
 // ptalog is the (optional) pointer-analysis log file.
 // buildContext is the go/build configuration for locating packages.
@@ -192,8 +192,11 @@ func (res *Result) Serial() *serial.Result {
 // Clients that intend to perform multiple queries against the same
 // analysis scope should use this pattern instead:
 //
-//	imp := importer.New(&importer.Config{Build: buildContext, SourceImports: true})
-// 	o, err := oracle.New(imp, args, nil)
+//	conf := importer.Config{Build: buildContext, SourceImports: true}
+//	... populate config, e.g. conf.FromArgs(args) ...
+//	iprog, err := conf.Load()
+//	if err != nil { ... }
+// 	o, err := oracle.New(iprog, nil, false)
 //	if err != nil { ... }
 //	for ... {
 //		qpos, err := oracle.ParseQueryPos(imp, pos, needExact)
@@ -219,29 +222,44 @@ func Query(args []string, mode, pos string, ptalog io.Writer, buildContext *buil
 		return nil, fmt.Errorf("invalid mode type: %q", mode)
 	}
 
-	impcfg := importer.Config{Build: buildContext, SourceImports: true}
+	conf := importer.Config{Build: buildContext, SourceImports: true}
+
+	// Determine initial packages.
+	args, err := conf.FromArgs(args)
+	if err != nil {
+		return nil, err
+	}
+	if len(args) > 0 {
+		return nil, fmt.Errorf("surplus arguments: %q", args)
+	}
 
 	// For queries needing only a single typed package,
 	// reduce the analysis scope to that package.
 	if minfo.needs&(needSSA|needRetainTypeInfo) == 0 {
-		reduceScope(pos, &impcfg, &args)
+		reduceScope(pos, &conf)
 	}
 
 	// TODO(adonovan): report type errors to the user via Serial
 	// types, not stderr?
-	// impcfg.TypeChecker.Error = func(err error) {
+	// conf.TypeChecker.Error = func(err error) {
 	// 	E := err.(types.Error)
 	// 	fmt.Fprintf(os.Stderr, "%s: %s\n", E.Fset.Position(E.Pos), E.Msg)
 	// }
-	imp := importer.New(&impcfg)
-	o, err := newOracle(imp, args, ptalog, minfo.needs, reflection)
+
+	// Load/parse/type-check the program.
+	iprog, err := conf.Load()
+	if err != nil {
+		return nil, err
+	}
+
+	o, err := newOracle(iprog, ptalog, minfo.needs, reflection)
 	if err != nil {
 		return nil, err
 	}
 
 	var qpos *QueryPos
 	if minfo.needs&(needPos|needExactPos) != 0 {
-		qpos, err = ParseQueryPos(imp, pos, minfo.needs&needExactPos != 0)
+		qpos, err = ParseQueryPos(iprog, pos, minfo.needs&needExactPos != 0)
 		if err != nil {
 			return nil, err
 		}
@@ -249,7 +267,7 @@ func Query(args []string, mode, pos string, ptalog io.Writer, buildContext *buil
 
 	// SSA is built and we have the QueryPos.
 	// Release the other ASTs and type info to the GC.
-	imp = nil
+	iprog = nil
 
 	return o.query(minfo, qpos)
 }
@@ -262,12 +280,7 @@ func Query(args []string, mode, pos string, ptalog io.Writer, buildContext *buil
 //
 // TODO(adonovan): this is a real mess... but it's fast.
 //
-func reduceScope(pos string, impcfg *importer.Config, args *[]string) {
-	// TODO(adonovan): make the 'args' argument of
-	// (*Importer).LoadInitialPackages part of the
-	// importer.Config, and inline LoadInitialPackages into
-	// NewImporter.  Then we won't need the 'args' argument.
-
+func reduceScope(pos string, conf *importer.Config) {
 	fqpos, err := fastQueryPos(pos)
 	if err != nil {
 		return // bad query
@@ -276,7 +289,7 @@ func reduceScope(pos string, impcfg *importer.Config, args *[]string) {
 	// TODO(adonovan): fix: this gives the wrong results for files
 	// in non-importable packages such as tests and ad-hoc packages
 	// specified as a list of files (incl. the oracle's tests).
-	_, importPath, err := guessImportPath(fqpos.fset.File(fqpos.start).Name(), impcfg.Build)
+	_, importPath, err := guessImportPath(fqpos.fset.File(fqpos.start).Name(), conf.Build)
 	if err != nil {
 		return // can't find GOPATH dir
 	}
@@ -287,9 +300,9 @@ func reduceScope(pos string, impcfg *importer.Config, args *[]string) {
 	// Check that it's possible to load the queried package.
 	// (e.g. oracle tests contain different 'package' decls in same dir.)
 	// Keep consistent with logic in importer/util.go!
-	ctxt2 := *impcfg.Build
-	ctxt2.CgoEnabled = false
-	bp, err := ctxt2.Import(importPath, "", 0)
+	cfg2 := *conf.Build
+	cfg2.CgoEnabled = false
+	bp, err := cfg2.Import(importPath, "", 0)
 	if err != nil {
 		return // no files for package
 	}
@@ -302,59 +315,50 @@ func reduceScope(pos string, impcfg *importer.Config, args *[]string) {
 	//  return // not found
 	// found:
 
-	impcfg.TypeCheckFuncBodies = func(p string) bool { return p == importPath }
-	*args = []string{importPath}
+	conf.TypeCheckFuncBodies = func(p string) bool { return p == importPath }
+
+	// Ignore packages specified on command line.
+	conf.CreatePkgs = nil
+	conf.ImportPkgs = nil
+
+	// Instead load just the one containing the query position
+	// (and possibly its corresponding tests/production code).
+	// TODO(adonovan): set 'augment' based on which file list
+	// contains
+	_ = conf.ImportWithTests(importPath) // ignore error
 }
 
 // New constructs a new Oracle that can be used for a sequence of queries.
 //
-// imp will be used to load source code for imported packages.
-// It must not yet have loaded any packages.
-//
-// args specify the main package in importer.LoadInitialPackages syntax.
-//
+// iprog specifies the program to analyze.
 // ptalog is the (optional) pointer-analysis log file.
 // reflection determines whether to model reflection soundly (currently slow).
 //
-func New(imp *importer.Importer, args []string, ptalog io.Writer, reflection bool) (*Oracle, error) {
-	return newOracle(imp, args, ptalog, needAll, reflection)
+func New(iprog *importer.Program, ptalog io.Writer, reflection bool) (*Oracle, error) {
+	return newOracle(iprog, ptalog, needAll, reflection)
 }
 
-func newOracle(imp *importer.Importer, args []string, ptalog io.Writer, needs int, reflection bool) (*Oracle, error) {
-	o := &Oracle{fset: imp.Fset}
-
-	// Load/parse/type-check program from args.
-	initialPkgInfos, args, err := imp.LoadInitialPackages(args)
-	if err != nil {
-		return nil, err // I/O or parser error
-	}
-	if len(args) > 0 {
-		return nil, fmt.Errorf("surplus arguments: %q", args)
-	}
+func newOracle(iprog *importer.Program, ptalog io.Writer, needs int, reflection bool) (*Oracle, error) {
+	o := &Oracle{fset: iprog.Fset}
 
 	// Retain type info for all ASTs in the program.
 	if needs&needRetainTypeInfo != 0 {
-		m := make(map[*types.Package]*importer.PackageInfo)
-		for _, p := range imp.AllPackages() {
-			m[p.Pkg] = p
-		}
-		o.typeInfo = m
+		o.typeInfo = iprog.AllPackages
 	}
 
 	// Create SSA package for the initial packages and their dependencies.
 	if needs&needSSA != 0 {
-		prog := ssa.NewProgram(o.fset, 0)
-
-		// Create SSA packages.
-		if err := prog.CreatePackages(imp); err != nil {
-			return nil, err
+		var mode ssa.BuilderMode
+		if needs&needSSADebug != 0 {
+			mode |= ssa.GlobalDebug
 		}
+		prog := ssa.Create(iprog, mode)
 
 		// For each initial package (specified on the command line),
 		// if it has a main function, analyze that,
 		// otherwise analyze its tests, if any.
 		var testPkgs, mains []*ssa.Package
-		for _, info := range initialPkgInfos {
+		for _, info := range iprog.InitialPackages() {
 			initialPkg := prog.Package(info.Pkg)
 
 			// Add package to the pointer analysis scope.
@@ -376,12 +380,6 @@ func newOracle(imp *importer.Importer, args []string, ptalog io.Writer, needs in
 		o.ptaConfig.Reflection = reflection
 		o.ptaConfig.Mains = mains
 
-		if needs&needSSADebug != 0 {
-			for _, pkg := range prog.AllPackages() {
-				pkg.SetDebugMode(true)
-			}
-		}
-
 		o.prog = prog
 	}
 
@@ -423,23 +421,23 @@ func (o *Oracle) query(minfo *modeInfo, qpos *QueryPos) (*Result, error) {
 // this is appropriate for queries that allow fairly arbitrary syntax,
 // e.g. "describe".
 //
-func ParseQueryPos(imp *importer.Importer, posFlag string, needExact bool) (*QueryPos, error) {
+func ParseQueryPos(iprog *importer.Program, posFlag string, needExact bool) (*QueryPos, error) {
 	filename, startOffset, endOffset, err := parsePosFlag(posFlag)
 	if err != nil {
 		return nil, err
 	}
-	start, end, err := findQueryPos(imp.Fset, filename, startOffset, endOffset)
+	start, end, err := findQueryPos(iprog.Fset, filename, startOffset, endOffset)
 	if err != nil {
 		return nil, err
 	}
-	info, path, exact := imp.PathEnclosingInterval(start, end)
+	info, path, exact := iprog.PathEnclosingInterval(start, end)
 	if path == nil {
 		return nil, fmt.Errorf("no syntax here")
 	}
 	if needExact && !exact {
 		return nil, fmt.Errorf("ambiguous selection within %s", astutil.NodeDescription(path[0]))
 	}
-	return &QueryPos{imp.Fset, start, end, path, exact, info}, nil
+	return &QueryPos{iprog.Fset, start, end, path, exact, info}, nil
 }
 
 // WriteTo writes the oracle query result res to out in a compiler diagnostic format.
diff --git a/oracle/oracle_test.go b/oracle/oracle_test.go
index ca7c9b22..2ab26973 100644
--- a/oracle/oracle_test.go
+++ b/oracle/oracle_test.go
@@ -260,24 +260,29 @@ func TestMultipleQueries(t *testing.T) {
 	// Importer
 	var buildContext = build.Default
 	buildContext.GOPATH = "testdata"
-	imp := importer.New(&importer.Config{Build: &buildContext})
+	conf := importer.Config{Build: &buildContext}
+	filename := "testdata/src/main/multi.go"
+	conf.CreateFromFilenames(filename)
+	iprog, err := conf.Load()
+	if err != nil {
+		t.Fatalf("Load failed: %s", err)
+	}
 
 	// Oracle
-	filename := "testdata/src/main/multi.go"
-	o, err := oracle.New(imp, []string{filename}, nil, true)
+	o, err := oracle.New(iprog, nil, true)
 	if err != nil {
 		t.Fatalf("oracle.New failed: %s", err)
 	}
 
 	// QueryPos
 	pos := filename + ":#54,#58"
-	qpos, err := oracle.ParseQueryPos(imp, pos, true)
+	qpos, err := oracle.ParseQueryPos(iprog, pos, true)
 	if err != nil {
 		t.Fatalf("oracle.ParseQueryPos(%q) failed: %s", pos, err)
 	}
 	// SSA is built and we have the QueryPos.
 	// Release the other ASTs and type info to the GC.
-	imp = nil
+	iprog = nil
 
 	// Run different query modes on same scope and selection.
 	out := new(bytes.Buffer)
diff --git a/pointer/example_test.go b/pointer/example_test.go
index 2af0c318..4b476b2a 100644
--- a/pointer/example_test.go
+++ b/pointer/example_test.go
@@ -6,7 +6,6 @@ package pointer_test
 
 import (
 	"fmt"
-	"go/parser"
 	"sort"
 
 	"code.google.com/p/go.tools/call"
@@ -40,26 +39,27 @@ func main() {
 }
 `
 	// Construct an importer.
-	imp := importer.New(&importer.Config{SourceImports: true})
+	conf := importer.Config{SourceImports: true}
 
 	// Parse the input file.
-	file, err := parser.ParseFile(imp.Fset, "myprog.go", myprog, 0)
+	file, err := conf.ParseFile("myprog.go", myprog, 0)
 	if err != nil {
 		fmt.Print(err) // parse error
 		return
 	}
 
 	// Create single-file main package and import its dependencies.
-	mainInfo := imp.CreatePackage("main", file)
+	conf.CreateFromFiles(file)
 
-	// Create SSA-form program representation.
-	var mode ssa.BuilderMode
-	prog := ssa.NewProgram(imp.Fset, mode)
-	if err := prog.CreatePackages(imp); err != nil {
+	iprog, err := conf.Load()
+	if err != nil {
 		fmt.Print(err) // type error in some package
 		return
 	}
-	mainPkg := prog.Package(mainInfo.Pkg)
+
+	// Create SSA-form program representation.
+	prog := ssa.Create(iprog, 0)
+	mainPkg := prog.Package(iprog.Created[0].Pkg)
 
 	// Build SSA code for bodies of all functions in the whole program.
 	prog.BuildAll()
diff --git a/pointer/pointer_test.go b/pointer/pointer_test.go
index de0e00da..36ba8707 100644
--- a/pointer/pointer_test.go
+++ b/pointer/pointer_test.go
@@ -151,30 +151,28 @@ func findProbe(prog *ssa.Program, probes map[*ssa.CallCommon]pointer.Pointer, e
 }
 
 func doOneInput(input, filename string) bool {
-	impctx := &importer.Config{SourceImports: true}
-	imp := importer.New(impctx)
+	conf := importer.Config{SourceImports: true}
 
 	// Parsing.
-	f, err := parser.ParseFile(imp.Fset, filename, input, 0)
+	f, err := conf.ParseFile(filename, input, 0)
 	if err != nil {
-		// TODO(adonovan): err is a scanner error list;
-		// display all errors not just first?
 		fmt.Println(err)
 		return false
 	}
 
 	// Create single-file main package and import its dependencies.
-	info := imp.CreatePackage("main", f)
-
-	// SSA creation + building.
-	prog := ssa.NewProgram(imp.Fset, ssa.SanityCheckFunctions)
-	if err := prog.CreatePackages(imp); err != nil {
+	conf.CreateFromFiles(f)
+	iprog, err := conf.Load()
+	if err != nil {
 		fmt.Println(err)
 		return false
 	}
+
+	// SSA creation + building.
+	prog := ssa.Create(iprog, ssa.SanityCheckFunctions)
 	prog.BuildAll()
 
-	mainpkg := prog.Package(info.Pkg)
+	mainpkg := prog.Package(iprog.Created[0].Pkg)
 	ptrmain := mainpkg // main package for the pointer analysis
 	if mainpkg.Func("main") == nil {
 		// No main function; assume it's a test.
@@ -228,7 +226,7 @@ func doOneInput(input, filename string) bool {
 							continue
 						}
 						mainFileScope := mainpkg.Object.Scope().Child(0)
-						t, _, err = types.EvalNode(imp.Fset, texpr, mainpkg.Object, mainFileScope)
+						t, _, err = types.EvalNode(prog.Fset, texpr, mainpkg.Object, mainFileScope)
 						if err != nil {
 							ok = false
 							// Don't print err since its location is bad.
diff --git a/pointer/testdata/a_test.go b/pointer/testdata/a_test.go
index 3baa9ac7..828dcb6f 100644
--- a/pointer/testdata/a_test.go
+++ b/pointer/testdata/a_test.go
@@ -11,7 +11,7 @@ import "testing"
 
 func log(f func(*testing.T)) {
 	// The PTS of f is the set of called tests.  TestingQuux is not present.
-	print(f) // @pointsto main.Test | main.TestFoo
+	print(f) // @pointsto a.Test | a.TestFoo
 }
 
 func Test(t *testing.T) {
@@ -36,7 +36,7 @@ func ExampleBar() {
 }
 
 // Excludes TestingQuux.
-// @calls testing.tRunner -> main.Test
-// @calls testing.tRunner -> main.TestFoo
-// @calls testing.runExample -> main.ExampleBar
-// @calls (*testing.B).runN -> main.BenchmarkFoo
+// @calls testing.tRunner -> a.Test
+// @calls testing.tRunner -> a.TestFoo
+// @calls testing.runExample -> a.ExampleBar
+// @calls (*testing.B).runN -> a.BenchmarkFoo
diff --git a/ssa/builder_test.go b/ssa/builder_test.go
index 757a1710..490e0854 100644
--- a/ssa/builder_test.go
+++ b/ssa/builder_test.go
@@ -5,7 +5,6 @@
 package ssa_test
 
 import (
-	"go/parser"
 	"reflect"
 	"sort"
 	"strings"
@@ -40,22 +39,24 @@ func main() {
         w.Write(nil)    // interface invoke of external declared method
 }
 `
-	imp := importer.New(&importer.Config{})
 
-	f, err := parser.ParseFile(imp.Fset, "<input>", test, 0)
+	// Create a single-file main package.
+	var conf importer.Config
+	f, err := conf.ParseFile("<input>", test, 0)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	conf.CreateFromFiles(f)
+
+	iprog, err := conf.Load()
 	if err != nil {
 		t.Error(err)
 		return
 	}
 
-	mainInfo := imp.CreatePackage("main", f)
-
-	prog := ssa.NewProgram(imp.Fset, ssa.SanityCheckFunctions)
-	if err := prog.CreatePackages(imp); err != nil {
-		t.Error(err)
-		return
-	}
-	mainPkg := prog.Package(mainInfo.Pkg)
+	prog := ssa.Create(iprog, ssa.SanityCheckFunctions)
+	mainPkg := prog.Package(iprog.Created[0].Pkg)
 	mainPkg.Build()
 
 	// The main package, its direct and indirect dependencies are loaded.
@@ -204,21 +205,22 @@ func TestTypesWithMethodSets(t *testing.T) {
 		},
 	}
 	for i, test := range tests {
-		imp := importer.New(&importer.Config{})
-
-		f, err := parser.ParseFile(imp.Fset, "<input>", test.input, 0)
+		// Create a single-file main package.
+		var conf importer.Config
+		f, err := conf.ParseFile("<input>", test.input, 0)
 		if err != nil {
 			t.Errorf("test %d: %s", i, err)
 			continue
 		}
+		conf.CreateFromFiles(f)
 
-		mainInfo := imp.CreatePackage("p", f)
-		prog := ssa.NewProgram(imp.Fset, ssa.SanityCheckFunctions)
-		if err := prog.CreatePackages(imp); err != nil {
-			t.Errorf("test %d: %s", i, err)
+		iprog, err := conf.Load()
+		if err != nil {
+			t.Errorf("test %d: Load: %s", i, err)
 			continue
 		}
-		mainPkg := prog.Package(mainInfo.Pkg)
+		prog := ssa.Create(iprog, ssa.SanityCheckFunctions)
+		mainPkg := prog.Package(iprog.Created[0].Pkg)
 		prog.BuildAll()
 
 		var typstrs []string
diff --git a/ssa/create.go b/ssa/create.go
index 951400f8..60b2986c 100644
--- a/ssa/create.go
+++ b/ssa/create.go
@@ -8,11 +8,9 @@ package ssa
 // See builder.go for explanation.
 
 import (
-	"fmt"
 	"go/ast"
 	"go/token"
 	"os"
-	"strings"
 
 	"code.google.com/p/go.tools/go/types"
 	"code.google.com/p/go.tools/importer"
@@ -28,25 +26,32 @@ const (
 	SanityCheckFunctions                         // Perform sanity checking of function bodies
 	NaiveForm                                    // Build naïve SSA form: don't replace local loads/stores with registers
 	BuildSerially                                // Build packages serially, not in parallel.
+	GlobalDebug                                  // Enable debug info for all packages
 )
 
-// NewProgram returns a new SSA Program initially containing no
-// packages.
+// Create returns a new SSA Program, creating all packages and all
+// their members.
 //
-// fset specifies the mapping from token positions to source location
-// that will be used by all ASTs of this program.
+// Code for bodies of functions is not built until Build() is called
+// on the result.
 //
 // mode controls diagnostics and checking during SSA construction.
 //
-func NewProgram(fset *token.FileSet, mode BuilderMode) *Program {
-	return &Program{
-		Fset:                fset,
+func Create(iprog *importer.Program, mode BuilderMode) *Program {
+	prog := &Program{
+		Fset:                iprog.Fset,
 		imported:            make(map[string]*Package),
 		packages:            make(map[*types.Package]*Package),
 		boundMethodWrappers: make(map[*types.Func]*Function),
 		ifaceMethodWrappers: make(map[*types.Func]*Function),
 		mode:                mode,
 	}
+
+	for _, info := range iprog.AllPackages {
+		prog.CreatePackage(info)
+	}
+
+	return prog
 }
 
 // memberFromObject populates package pkg with a member for the
@@ -166,9 +171,6 @@ func membersFromDecl(pkg *Package, decl ast.Decl) {
 // until a subsequent call to Package.Build().
 //
 func (prog *Program) CreatePackage(info *importer.PackageInfo) *Package {
-	if info.Err != nil {
-		panic(fmt.Sprintf("package %s has errors: %s", info, info.Err))
-	}
 	if p := prog.packages[info.Pkg]; p != nil {
 		return p // already loaded
 	}
@@ -225,6 +227,10 @@ func (prog *Program) CreatePackage(info *importer.PackageInfo) *Package {
 	}
 	p.Members[initguard.Name()] = initguard
 
+	if prog.mode&GlobalDebug != 0 {
+		p.SetDebugMode(true)
+	}
+
 	if prog.mode&LogPackages != 0 {
 		p.DumpTo(os.Stderr)
 	}
@@ -241,40 +247,6 @@ func (prog *Program) CreatePackage(info *importer.PackageInfo) *Package {
 	return p
 }
 
-// CreatePackages creates SSA Packages for all error-free packages
-// loaded by the specified Importer.
-//
-// If all packages were error-free, it is safe to call
-// prog.BuildAll(), and nil is returned.  Otherwise an error is
-// returned.
-//
-func (prog *Program) CreatePackages(imp *importer.Importer) error {
-	var errpkgs []string
-
-	// Create source packages and directly imported packages.
-	for _, info := range imp.AllPackages() {
-		if info.Err != nil {
-			errpkgs = append(errpkgs, info.Pkg.Path())
-		} else {
-			prog.CreatePackage(info)
-		}
-	}
-
-	// Create indirectly imported packages.
-	for _, obj := range imp.Config.TypeChecker.Packages {
-		prog.CreatePackage(&importer.PackageInfo{
-			Pkg:        obj,
-			Importable: true,
-		})
-	}
-
-	if errpkgs != nil {
-		return fmt.Errorf("couldn't create these SSA packages due to type errors: %s",
-			strings.Join(errpkgs, ", "))
-	}
-	return nil
-}
-
 // AllPackages returns a new slice containing all packages in the
 // program prog in unspecified order.
 //
diff --git a/ssa/doc.go b/ssa/doc.go
index 878addba..82047875 100644
--- a/ssa/doc.go
+++ b/ssa/doc.go
@@ -23,17 +23,11 @@
 // primitives in the future to facilitate constant-time dispatch of
 // switch statements, for example.
 //
-// Builder encapsulates the tasks of type-checking (using go/types)
-// abstract syntax trees (as defined by go/ast) for the source files
-// comprising a Go program, and the conversion of each function from
-// Go ASTs to the SSA representation.
-//
-// By supplying an instance of the SourceLocator function prototype,
-// clients may control how the builder locates, loads and parses Go
-// sources files for imported packages.  This package provides
-// MakeGoBuildLoader, which creates a loader that uses go/build to
-// locate packages in the Go source distribution, and go/parser to
-// parse them.
+// To construct an SSA-form program, call ssa.Create on an
+// importer.Program, a set of type-checked packages created from
+// parsed Go source files.  The resulting ssa.Program contains all the
+// packages and their members, but SSA code is not created for
+// function bodies until a subsequent call to (*Package).Build.
 //
 // The builder initially builds a naive SSA form in which all local
 // variables are addresses of stack locations with explicit loads and
diff --git a/ssa/example_test.go b/ssa/example_test.go
index a9d98c94..20383ad3 100644
--- a/ssa/example_test.go
+++ b/ssa/example_test.go
@@ -6,7 +6,6 @@ package ssa_test
 
 import (
 	"fmt"
-	"go/parser"
 	"os"
 
 	"code.google.com/p/go.tools/importer"
@@ -41,27 +40,28 @@ func main() {
 	fmt.Println(message)
 }
 `
-	// Construct an importer.
-	imp := importer.New(&importer.Config{})
+	var conf importer.Config
 
 	// Parse the input file.
-	file, err := parser.ParseFile(imp.Fset, "hello.go", hello, 0)
+	file, err := conf.ParseFile("hello.go", hello, 0)
 	if err != nil {
 		fmt.Print(err) // parse error
 		return
 	}
 
-	// Create single-file main package and import its dependencies.
-	mainInfo := imp.CreatePackage("main", file)
+	// Create single-file main package.
+	conf.CreateFromFiles(file)
 
-	// Create SSA-form program representation.
-	var mode ssa.BuilderMode
-	prog := ssa.NewProgram(imp.Fset, mode)
-	if err := prog.CreatePackages(imp); err != nil {
+	// Load the main package and its dependencies.
+	iprog, err := conf.Load()
+	if err != nil {
 		fmt.Print(err) // type error in some package
 		return
 	}
-	mainPkg := prog.Package(mainInfo.Pkg)
+
+	// Create SSA-form program representation.
+	prog := ssa.Create(iprog, ssa.SanityCheckFunctions)
+	mainPkg := prog.Package(iprog.Created[0].Pkg)
 
 	// Print out the package.
 	mainPkg.DumpTo(os.Stdout)
diff --git a/ssa/interp/interp_test.go b/ssa/interp/interp_test.go
index e712f999..515db619 100644
--- a/ssa/interp/interp_test.go
+++ b/ssa/interp/interp_test.go
@@ -168,16 +168,16 @@ func run(t *testing.T, dir, input string, success successPredicate) bool {
 		inputs = append(inputs, dir+i)
 	}
 
-	imp := importer.New(&importer.Config{SourceImports: true})
-	// TODO(adonovan): use LoadInitialPackages, then un-export ParseFiles.
-	// Then add the following packages' tests, which pass:
+	conf := importer.Config{SourceImports: true}
+	// TODO(adonovan): add the following packages' tests, which pass:
 	// "flag", "unicode", "unicode/utf8", "testing", "log", "path".
-	files, err := importer.ParseFiles(imp.Fset, ".", inputs...)
-	if err != nil {
-		t.Errorf("ssa.ParseFiles(%s) failed: %s", inputs, err.Error())
+	if err := conf.CreateFromFilenames(inputs...); err != nil {
+		t.Errorf("CreateFromFilenames(%s) failed: %s", inputs, err)
 		return false
 	}
 
+	conf.Import("runtime")
+
 	// Print a helpful hint if we don't make it to the end.
 	var hint string
 	defer func() {
@@ -192,20 +192,17 @@ func run(t *testing.T, dir, input string, success successPredicate) bool {
 	}()
 
 	hint = fmt.Sprintf("To dump SSA representation, run:\n%% go build code.google.com/p/go.tools/cmd/ssadump && ./ssadump -build=CFP %s\n", input)
-	mainInfo := imp.CreatePackage(files[0].Name.Name, files...)
 
-	if _, err := imp.ImportPackage("runtime"); err != nil {
-		t.Errorf("ImportPackage(runtime) failed: %s", err)
-	}
-
-	prog := ssa.NewProgram(imp.Fset, ssa.SanityCheckFunctions)
-	if err := prog.CreatePackages(imp); err != nil {
-		t.Errorf("CreatePackages failed: %s", err)
+	iprog, err := conf.Load()
+	if err != nil {
+		t.Errorf("conf.Load(%s) failed: %s", inputs, err)
 		return false
 	}
+
+	prog := ssa.Create(iprog, ssa.SanityCheckFunctions)
 	prog.BuildAll()
 
-	mainPkg := prog.Package(mainInfo.Pkg)
+	mainPkg := prog.Package(iprog.Created[0].Pkg)
 	if mainPkg.Func("main") == nil {
 		testmainPkg := prog.CreateTestMainPackage(mainPkg)
 		if testmainPkg == nil {
@@ -321,17 +318,16 @@ func TestTestmainPackage(t *testing.T) {
 
 // CreateTestMainPackage should return nil if there were no tests.
 func TestNullTestmainPackage(t *testing.T) {
-	imp := importer.New(&importer.Config{})
-	files, err := importer.ParseFiles(imp.Fset, ".", "testdata/b_test.go")
-	if err != nil {
-		t.Fatalf("ParseFiles failed: %s", err)
+	var conf importer.Config
+	if err := conf.CreateFromFilenames("testdata/b_test.go"); err != nil {
+		t.Fatalf("ParseFile failed: %s", err)
 	}
-	mainInfo := imp.CreatePackage("b", files...)
-	prog := ssa.NewProgram(imp.Fset, ssa.SanityCheckFunctions)
-	if err := prog.CreatePackages(imp); err != nil {
+	iprog, err := conf.Load()
+	if err != nil {
 		t.Fatalf("CreatePackages failed: %s", err)
 	}
-	mainPkg := prog.Package(mainInfo.Pkg)
+	prog := ssa.Create(iprog, ssa.SanityCheckFunctions)
+	mainPkg := prog.Package(iprog.Created[0].Pkg)
 	if mainPkg.Func("main") != nil {
 		t.Fatalf("unexpected main function")
 	}
diff --git a/ssa/source_test.go b/ssa/source_test.go
index e3d94b8f..17ba439c 100644
--- a/ssa/source_test.go
+++ b/ssa/source_test.go
@@ -24,12 +24,13 @@ import (
 )
 
 func TestObjValueLookup(t *testing.T) {
-	imp := importer.New(&importer.Config{})
-	f, err := parser.ParseFile(imp.Fset, "testdata/objlookup.go", nil, parser.ParseComments)
+	var conf importer.Config
+	f, err := conf.ParseFile("testdata/objlookup.go", nil, parser.ParseComments)
 	if err != nil {
 		t.Error(err)
 		return
 	}
+	conf.CreateFromFiles(f)
 
 	// Maps each var Ident (represented "name:linenum") to the
 	// kind of ssa.Value we expect (represented "Constant", "&Alloc").
@@ -39,7 +40,7 @@ func TestObjValueLookup(t *testing.T) {
 	re := regexp.MustCompile(`(\b|&)?(\w*)::(\w*)\b`)
 	for _, c := range f.Comments {
 		text := c.Text()
-		pos := imp.Fset.Position(c.Pos())
+		pos := conf.Fset.Position(c.Pos())
 		for _, m := range re.FindAllStringSubmatch(text, -1) {
 			key := fmt.Sprintf("%s:%d", m[2], pos.Line)
 			value := m[1] + m[3]
@@ -47,13 +48,14 @@ func TestObjValueLookup(t *testing.T) {
 		}
 	}
 
-	mainInfo := imp.CreatePackage("main", f)
-
-	prog := ssa.NewProgram(imp.Fset, 0 /*|ssa.LogFunctions*/)
-	if err := prog.CreatePackages(imp); err != nil {
+	iprog, err := conf.Load()
+	if err != nil {
 		t.Error(err)
 		return
 	}
+
+	prog := ssa.Create(iprog, 0 /*|ssa.LogFunctions*/)
+	mainInfo := iprog.Created[0]
 	mainPkg := prog.Package(mainInfo.Pkg)
 	mainPkg.SetDebugMode(true)
 	mainPkg.Build()
@@ -90,7 +92,7 @@ func TestObjValueLookup(t *testing.T) {
 		}
 		if obj, ok := mainInfo.ObjectOf(id).(*types.Var); ok {
 			ref, _ := astutil.PathEnclosingInterval(f, id.Pos(), id.Pos())
-			pos := imp.Fset.Position(id.Pos())
+			pos := prog.Fset.Position(id.Pos())
 			exp := expectations[fmt.Sprintf("%s:%d", id.Name, pos.Line)]
 			if exp == "" {
 				t.Errorf("%s: no expectation for var ident %s ", pos, id.Name)
@@ -186,20 +188,23 @@ func checkVarValue(t *testing.T, prog *ssa.Program, pkg *ssa.Package, ref []ast.
 // Ensure that, in debug mode, we can determine the ssa.Value
 // corresponding to every ast.Expr.
 func TestValueForExpr(t *testing.T) {
-	imp := importer.New(&importer.Config{})
-	f, err := parser.ParseFile(imp.Fset, "testdata/valueforexpr.go", nil, parser.ParseComments)
+	var conf importer.Config
+	f, err := conf.ParseFile("testdata/valueforexpr.go", nil, parser.ParseComments)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	conf.CreateFromFiles(f)
+
+	iprog, err := conf.Load()
 	if err != nil {
 		t.Error(err)
 		return
 	}
 
-	mainInfo := imp.CreatePackage("main", f)
+	mainInfo := iprog.Created[0]
 
-	prog := ssa.NewProgram(imp.Fset, 0)
-	if err := prog.CreatePackages(imp); err != nil {
-		t.Error(err)
-		return
-	}
+	prog := ssa.Create(iprog, 0)
 	mainPkg := prog.Package(mainInfo.Pkg)
 	mainPkg.SetDebugMode(true)
 	mainPkg.Build()
@@ -232,7 +237,7 @@ func TestValueForExpr(t *testing.T) {
 		}
 		text = text[1:]
 		pos := c.End() + 1
-		position := imp.Fset.Position(pos)
+		position := prog.Fset.Position(pos)
 		var e ast.Expr
 		if target := parenExprByPos[pos]; target == nil {
 			t.Errorf("%s: annotation doesn't precede ParenExpr: %q", position, text)
diff --git a/ssa/ssa.go b/ssa/ssa.go
index 7c2c7229..111af694 100644
--- a/ssa/ssa.go
+++ b/ssa/ssa.go
@@ -1170,7 +1170,8 @@ type MapUpdate struct {
 // For non-Ident expressions, Object() returns nil.
 //
 // DebugRefs are generated only for functions built with debugging
-// enabled; see Package.SetDebugMode().
+// enabled; see Package.SetDebugMode() and the GlobalDebug builder
+// mode flag.
 //
 // DebugRefs are not emitted for ast.Idents referring to constants or
 // predeclared identifiers, since they are trivial and numerous.
@@ -1240,7 +1241,7 @@ type anInstruction struct {
 //    (b) a *MakeClosure, indicating an immediately applied
 //        function literal with free variables.
 //    (c) a *Builtin, indicating a statically dispatched call
-//        to a built-in function.  StaticCallee returns nil.
+//        to a built-in function.
 //    (d) any other value, indicating a dynamically dispatched
 //        function call.
 // StaticCallee returns the identity of the callee in cases
diff --git a/ssa/ssautil/switch_test.go b/ssa/ssautil/switch_test.go
index b7cce89f..c41a3f28 100644
--- a/ssa/ssautil/switch_test.go
+++ b/ssa/ssautil/switch_test.go
@@ -15,21 +15,22 @@ import (
 )
 
 func TestSwitches(t *testing.T) {
-	imp := importer.New(&importer.Config{})
-	f, err := parser.ParseFile(imp.Fset, "testdata/switches.go", nil, parser.ParseComments)
+	var conf importer.Config
+	f, err := conf.ParseFile("testdata/switches.go", nil, parser.ParseComments)
 	if err != nil {
 		t.Error(err)
 		return
 	}
 
-	mainInfo := imp.CreatePackage("main", f)
-
-	prog := ssa.NewProgram(imp.Fset, 0)
-	if err := prog.CreatePackages(imp); err != nil {
+	conf.CreateFromFiles(f)
+	iprog, err := conf.Load()
+	if err != nil {
 		t.Error(err)
 		return
 	}
-	mainPkg := prog.Package(mainInfo.Pkg)
+
+	prog := ssa.Create(iprog, 0)
+	mainPkg := prog.Package(iprog.Created[0].Pkg)
 	mainPkg.Build()
 
 	for _, mem := range mainPkg.Members {
diff --git a/ssa/stdlib_test.go b/ssa/stdlib_test.go
index ac5bd6fa..f95e3669 100644
--- a/ssa/stdlib_test.go
+++ b/ssa/stdlib_test.go
@@ -23,8 +23,6 @@ import (
 	"code.google.com/p/go.tools/ssa/ssautil"
 )
 
-const debugMode = true // cost: +30% space, +18% time for SSA building
-
 func allPackages() []string {
 	var pkgs []string
 	root := filepath.Join(runtime.GOROOT(), "src/pkg") + string(os.PathSeparator)
@@ -54,13 +52,17 @@ func TestStdlib(t *testing.T) {
 	// Load, parse and type-check the program.
 	t0 := time.Now()
 
-	imp := importer.New(&importer.Config{})
-
-	if _, _, err := imp.LoadInitialPackages(allPackages()); err != nil {
-		t.Errorf("LoadInitialPackages failed: %s", err)
+	var conf importer.Config
+	if _, err := conf.FromArgs(allPackages()); err != nil {
+		t.Errorf("FromArgs failed: %s", err)
 		return
 	}
 
+	iprog, err := conf.Load()
+	if err != nil {
+		t.Errorf("Load failed: %s", err)
+	}
+
 	t1 := time.Now()
 
 	runtime.GC()
@@ -69,15 +71,11 @@ func TestStdlib(t *testing.T) {
 	alloc := memstats.Alloc
 
 	// Create SSA packages.
-	prog := ssa.NewProgram(imp.Fset, ssa.SanityCheckFunctions)
-	if err := prog.CreatePackages(imp); err != nil {
-		t.Errorf("CreatePackages failed: %s", err)
-		return
-	}
-	// Enable debug mode globally.
-	for _, info := range imp.AllPackages() {
-		prog.Package(info.Pkg).SetDebugMode(debugMode)
-	}
+	var mode ssa.BuilderMode
+	// Comment out these lines during benchmarking.  Approx SSA build costs are noted.
+	mode |= ssa.SanityCheckFunctions // + 2% space, + 4% time
+	mode |= ssa.GlobalDebug          // +30% space, +18% time
+	prog := ssa.Create(iprog, mode)
 
 	t2 := time.Now()
 
@@ -105,7 +103,7 @@ func TestStdlib(t *testing.T) {
 
 	// determine line count
 	var lineCount int
-	imp.Fset.Iterate(func(f *token.File) bool {
+	prog.Fset.Iterate(func(f *token.File) bool {
 		lineCount += f.LineCount()
 		return true
 	})