diff --git a/cmd/callgraph/main_test.go b/cmd/callgraph/main_test.go
index 1d286193..54953c70 100644
--- a/cmd/callgraph/main_test.go
+++ b/cmd/callgraph/main_test.go
@@ -12,11 +12,25 @@ package main
 import (
 	"bytes"
 	"fmt"
+	"log"
+	"os"
 	"path/filepath"
 	"strings"
 	"testing"
 )
 
+func init() {
+	// This test currently requires GOPATH mode.
+	// Explicitly disabling module mode should suffix, but
+	// we'll also turn off GOPROXY just for good measure.
+	if err := os.Setenv("GO111MODULE", "off"); err != nil {
+		log.Fatal(err)
+	}
+	if err := os.Setenv("GOPROXY", "off"); err != nil {
+		log.Fatal(err)
+	}
+}
+
 func TestCallgraph(t *testing.T) {
 	gopath, err := filepath.Abs("testdata")
 	if err != nil {
diff --git a/cmd/fiximports/main.go b/cmd/fiximports/main.go
index 365a2614..7de018fd 100644
--- a/cmd/fiximports/main.go
+++ b/cmd/fiximports/main.go
@@ -491,15 +491,18 @@ func list(args ...string) ([]*listPackage, error) {
 	return pkgs, nil
 }
 
-var cwd string
-
-func init() {
-	var err error
-	cwd, err = os.Getwd()
+// cwd contains the current working directory of the tool.
+//
+// It is initialized directly so that its value will be set for any other
+// package variables or init functions that depend on it, such as the gopath
+// variable in main_test.go.
+var cwd string = func() string {
+	cwd, err := os.Getwd()
 	if err != nil {
 		log.Fatalf("os.Getwd: %v", err)
 	}
-}
+	return cwd
+}()
 
 // shortPath returns an absolute or relative name for path, whatever is shorter.
 // Plundered from $GOROOT/src/cmd/go/build.go.
diff --git a/cmd/fiximports/main_test.go b/cmd/fiximports/main_test.go
index 75319a02..a2973a39 100644
--- a/cmd/fiximports/main_test.go
+++ b/cmd/fiximports/main_test.go
@@ -10,6 +10,7 @@ package main
 
 import (
 	"bytes"
+	"log"
 	"os"
 	"path/filepath"
 	"runtime"
@@ -32,11 +33,25 @@ import (
 // titanic.biz/bar	-- domain is sinking; package has jumped ship to new.com/bar
 // titanic.biz/foo	-- domain is sinking but package has no import comment yet
 
-func TestFixImports(t *testing.T) {
-	gopath := filepath.Join(cwd, "testdata")
+var gopath = filepath.Join(cwd, "testdata")
+
+func init() {
 	if err := os.Setenv("GOPATH", gopath); err != nil {
-		t.Fatalf("os.Setenv: %v", err)
+		log.Fatal(err)
 	}
+
+	// This test currently requires GOPATH mode.
+	// Explicitly disabling module mode should suffix, but
+	// we'll also turn off GOPROXY just for good measure.
+	if err := os.Setenv("GO111MODULE", "off"); err != nil {
+		log.Fatal(err)
+	}
+	if err := os.Setenv("GOPROXY", "off"); err != nil {
+		log.Fatal(err)
+	}
+}
+
+func TestFixImports(t *testing.T) {
 	defer func() {
 		stderr = os.Stderr
 		*badDomains = "code.google.com"
@@ -224,11 +239,6 @@ import (
 
 // TestDryRun tests that the -n flag suppresses calls to writeFile.
 func TestDryRun(t *testing.T) {
-	gopath := filepath.Join(cwd, "testdata")
-	if err := os.Setenv("GOPATH", gopath); err != nil {
-		t.Fatalf("os.Setenv: %v", err)
-	}
-
 	*dryrun = true
 	defer func() { *dryrun = false }() // restore
 	stderr = new(bytes.Buffer)
diff --git a/cmd/godoc/godoc_test.go b/cmd/godoc/godoc_test.go
index ab12353d..7200c917 100644
--- a/cmd/godoc/godoc_test.go
+++ b/cmd/godoc/godoc_test.go
@@ -233,10 +233,14 @@ func testWeb(t *testing.T, withIndex bool) {
 	cmd.Stderr = os.Stderr
 	cmd.Args[0] = "godoc"
 
-	// Set GOPATH variable to non-existing path.
+	// Set GOPATH variable to non-existing path
+	// and GOPROXY=off to disable module fetches.
 	// We cannot just unset GOPATH variable because godoc would default it to ~/go.
 	// (We don't want the indexer looking at the local workspace during tests.)
-	cmd.Env = append(os.Environ(), "GOPATH=does_not_exist")
+	cmd.Env = append(os.Environ(),
+		"GOPATH=does_not_exist",
+		"GOPROXY=off",
+		"GO111MODULE=off")
 
 	if err := cmd.Start(); err != nil {
 		t.Fatalf("failed to start godoc: %s", err)
@@ -448,6 +452,8 @@ func main() { print(lib.V) }
 	cmd.Env = os.Environ()
 	cmd.Env = append(cmd.Env, fmt.Sprintf("GOROOT=%s", filepath.Join(tmpdir, "goroot")))
 	cmd.Env = append(cmd.Env, fmt.Sprintf("GOPATH=%s", filepath.Join(tmpdir, "gopath")))
+	cmd.Env = append(cmd.Env, "GO111MODULE=off")
+	cmd.Env = append(cmd.Env, "GOPROXY=off")
 	cmd.Stdout = os.Stderr
 	stderr, err := cmd.StderrPipe()
 	if err != nil {
diff --git a/cmd/guru/guru_test.go b/cmd/guru/guru_test.go
index 5a06d09c..6322275d 100644
--- a/cmd/guru/guru_test.go
+++ b/cmd/guru/guru_test.go
@@ -35,6 +35,7 @@ import (
 	"go/token"
 	"io"
 	"io/ioutil"
+	"log"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -49,6 +50,18 @@ import (
 	guru "golang.org/x/tools/cmd/guru"
 )
 
+func init() {
+	// This test currently requires GOPATH mode.
+	// Explicitly disabling module mode should suffix, but
+	// we'll also turn off GOPROXY just for good measure.
+	if err := os.Setenv("GO111MODULE", "off"); err != nil {
+		log.Fatal(err)
+	}
+	if err := os.Setenv("GOPROXY", "off"); err != nil {
+		log.Fatal(err)
+	}
+}
+
 var updateFlag = flag.Bool("update", false, "Update the golden files.")
 
 type query struct {
diff --git a/go/analysis/passes/findcall/findcall_test.go b/go/analysis/passes/findcall/findcall_test.go
index c521885c..105f1a0e 100644
--- a/go/analysis/passes/findcall/findcall_test.go
+++ b/go/analysis/passes/findcall/findcall_test.go
@@ -1,12 +1,26 @@
 package findcall_test
 
 import (
+	"log"
+	"os"
 	"testing"
 
 	"golang.org/x/tools/go/analysis/analysistest"
 	"golang.org/x/tools/go/analysis/passes/findcall"
 )
 
+func init() {
+	// This test currently requires GOPATH mode.
+	// Explicitly disabling module mode should suffix, but
+	// we'll also turn off GOPROXY just for good measure.
+	if err := os.Setenv("GO111MODULE", "off"); err != nil {
+		log.Fatal(err)
+	}
+	if err := os.Setenv("GOPROXY", "off"); err != nil {
+		log.Fatal(err)
+	}
+}
+
 // TestFromStringLiterals demonstrates how to test an analysis using
 // a table of string literals for each test case.
 //
diff --git a/go/analysis/passes/pkgfact/pkgfact_test.go b/go/analysis/passes/pkgfact/pkgfact_test.go
index 11e6f893..511026c0 100644
--- a/go/analysis/passes/pkgfact/pkgfact_test.go
+++ b/go/analysis/passes/pkgfact/pkgfact_test.go
@@ -1,12 +1,26 @@
 package pkgfact_test
 
 import (
+	"log"
+	"os"
 	"testing"
 
 	"golang.org/x/tools/go/analysis/analysistest"
 	"golang.org/x/tools/go/analysis/passes/pkgfact"
 )
 
+func init() {
+	// This test currently requires GOPATH mode.
+	// Explicitly disabling module mode should suffix, but
+	// we'll also turn off GOPROXY just for good measure.
+	if err := os.Setenv("GO111MODULE", "off"); err != nil {
+		log.Fatal(err)
+	}
+	if err := os.Setenv("GOPROXY", "off"); err != nil {
+		log.Fatal(err)
+	}
+}
+
 func Test(t *testing.T) {
 	testdata := analysistest.TestData()
 	analysistest.Run(t, testdata, pkgfact.Analyzer,