diff --git a/cmd/digraph/digraph.go b/cmd/digraph/digraph.go
new file mode 100644
index 00000000..0d3868c4
--- /dev/null
+++ b/cmd/digraph/digraph.go
@@ -0,0 +1,411 @@
+// The digraph command performs queries over unlabelled directed graphs
+// represented in text form.  It is intended to integrate nicely with
+// typical UNIX command pipelines.
+//
+// Since directed graphs (import graphs, reference graphs, call graphs,
+// etc) often arise during software tool development and debugging, this
+// command is included in the go.tools repository.
+//
+// TODO(adonovan):
+// - support input files other than stdin
+// - suport alternative formats (AT&T GraphViz, CSV, etc),
+//   a comment syntax, etc.
+// - allow queries to nest, like Blaze query language.
+//
+package main
+
+import (
+	"bufio"
+	"flag"
+	"fmt"
+	"io"
+	"os"
+	"sort"
+	"strings"
+)
+
+const Usage = `digraph: queries over directed graphs in text form.
+
+Graph format:
+
+  Each line contains zero or more whitespace-separated fields.
+  Each field declares a node, and if there are more than one,
+  an edge from the first to each subsequent one.
+  The graph is provided on the standard input.
+
+  For instance, the following (acyclic) graph specifies a partial order
+  among the subtasks of getting dressed:
+
+	% cat clothes.txt
+	socks shoes
+	shorts pants
+	pants belt shoes
+	shirt tie sweater
+	sweater jacket
+	hat
+
+  The line "shirt tie sweater" indicates the two edges shirt -> tie and
+  shirt -> sweater, not shirt -> tie -> sweater.
+
+Supported queries:
+
+  nodes
+	the set of all nodes
+  degree
+	the in-degree and out-degree of each node.
+  preds <label> ...
+	the set of immediate predecessors of the specified nodes
+  succs <label> ...
+	the set of immediate successors of the specified nodes
+  forward <label> ...
+	the set of nodes transitively reachable from the specified nodes
+  reverse <label> ...
+	the set of nodes that transitively reach the specified nodes
+  somepath <label> <label>
+	the list of nodes on some arbitrary path from the first node to the second
+  allpaths <label> <label>
+	the set of nodes on all paths from the first node to the second
+  sccs
+	all strongly connected components (one per line)
+  scc <label>
+	the set of nodes nodes strongly connected to the specified one
+
+Example usage:
+
+   Show the transitive closure of imports of the digraph tool itself:
+   % go list -f '{{.ImportPath}}{{.Imports}}' ... | tr '[]' '  ' |
+         digraph forward code.google.com/p/go.tools/cmd/digraph
+
+   Show which clothes (see above) must be donned before a jacket:
+   %  digraph reverse jacket <clothes.txt
+
+`
+
+func main() {
+	flag.Parse()
+
+	args := flag.Args()
+	if len(args) == 0 {
+		fmt.Println(Usage)
+		return
+	}
+
+	if err := digraph(args[0], args[1:]); err != nil {
+		fmt.Fprintf(os.Stderr, "Error: %s\n", err)
+		os.Exit(1)
+	}
+}
+
+type nodelist []string
+
+func (l nodelist) println(sep string) {
+	for i, label := range l {
+		if i > 0 {
+			fmt.Fprint(stdout, sep)
+		}
+		fmt.Fprint(stdout, label)
+	}
+	fmt.Fprintln(stdout)
+}
+
+type nodeset map[string]bool
+
+func (s nodeset) sort() nodelist {
+	labels := make(nodelist, len(s))
+	var i int
+	for label := range s {
+		labels[i] = label
+		i++
+	}
+	sort.Strings(labels)
+	return labels
+}
+
+func (s nodeset) addAll(x nodeset) {
+	for label := range x {
+		s[label] = true
+	}
+}
+
+// A graph maps nodes to the non-nil set of their immediate successors.
+type graph map[string]nodeset
+
+func (g graph) addNode(label string) nodeset {
+	edges := g[label]
+	if edges == nil {
+		edges = make(nodeset)
+		g[label] = edges
+	}
+	return edges
+}
+
+func (g graph) addEdges(from string, to ...string) {
+	edges := g.addNode(from)
+	for _, to := range to {
+		g.addNode(to)
+		edges[to] = true
+	}
+}
+
+func (g graph) reachableFrom(roots nodeset) nodeset {
+	seen := make(nodeset)
+	var visit func(label string)
+	visit = func(label string) {
+		if !seen[label] {
+			seen[label] = true
+			for e := range g[label] {
+				visit(e)
+			}
+		}
+	}
+	for root := range roots {
+		visit(root)
+	}
+	return seen
+}
+
+func (g graph) transpose() graph {
+	rev := make(graph)
+	for label, edges := range g {
+		rev.addNode(label)
+		for succ := range edges {
+			rev.addEdges(succ, label)
+		}
+	}
+	return rev
+}
+
+func (g graph) sccs() []nodeset {
+	// Kosaraju's algorithm---Tarjan is overkill here.
+
+	// Forward pass.
+	S := make(nodelist, 0, len(g)) // postorder stack
+	seen := make(nodeset)
+	var visit func(label string)
+	visit = func(label string) {
+		if !seen[label] {
+			seen[label] = true
+			for e := range g[label] {
+				visit(e)
+			}
+			S = append(S, label)
+		}
+	}
+	for label := range g {
+		visit(label)
+	}
+
+	// Reverse pass.
+	rev := g.transpose()
+	var scc nodeset
+	seen = make(nodeset)
+	var rvisit func(label string)
+	rvisit = func(label string) {
+		if !seen[label] {
+			seen[label] = true
+			scc[label] = true
+			for e := range rev[label] {
+				rvisit(e)
+			}
+		}
+	}
+	var sccs []nodeset
+	for len(S) > 0 {
+		top := S[len(S)-1]
+		S = S[:len(S)-1] // pop
+		if !seen[top] {
+			scc = make(nodeset)
+			rvisit(top)
+			sccs = append(sccs, scc)
+		}
+	}
+	return sccs
+}
+
+func parse(rd io.Reader) (graph, error) {
+	g := make(graph)
+
+	in := bufio.NewScanner(rd)
+	for in.Scan() {
+		words := strings.Fields(in.Text())
+		if len(words) > 0 {
+			g.addEdges(words[0], words[1:]...)
+		}
+	}
+	if err := in.Err(); err != nil {
+		return nil, err
+	}
+	return g, nil
+}
+
+var stdin io.Reader = os.Stdin
+var stdout io.Writer = os.Stdout
+
+func digraph(cmd string, args []string) error {
+	// Parse the input graph.
+	g, err := parse(stdin)
+	if err != nil {
+		return err
+	}
+
+	// Parse the command line.
+	switch cmd {
+	case "nodes":
+		if len(args) != 0 {
+			return fmt.Errorf("usage: nodes")
+		}
+		nodes := make(nodeset)
+		for label := range g {
+			nodes[label] = true
+		}
+		nodes.sort().println("\n")
+
+	case "degree":
+		if len(args) != 0 {
+			return fmt.Errorf("usage: degree")
+		}
+		nodes := make(nodeset)
+		for label := range g {
+			nodes[label] = true
+		}
+		rev := g.transpose()
+		for _, label := range nodes.sort() {
+			fmt.Fprintf(stdout, "%d\t%d\t%s\n", len(rev[label]), len(g[label]), label)
+		}
+
+	case "succs", "preds":
+		if len(args) == 0 {
+			return fmt.Errorf("usage: %s <label> ...", cmd)
+		}
+		g := g
+		if cmd == "preds" {
+			g = g.transpose()
+		}
+		result := make(nodeset)
+		for _, root := range args {
+			edges := g[root]
+			if edges == nil {
+				return fmt.Errorf("no such node %q", root)
+			}
+			result.addAll(edges)
+		}
+		result.sort().println("\n")
+
+	case "forward", "reverse":
+		if len(args) == 0 {
+			return fmt.Errorf("usage: %s <label> ...", cmd)
+		}
+		roots := make(nodeset)
+		for _, root := range args {
+			if g[root] == nil {
+				return fmt.Errorf("no such node %q", root)
+			}
+			roots[root] = true
+		}
+		g := g
+		if cmd == "reverse" {
+			g = g.transpose()
+		}
+		g.reachableFrom(roots).sort().println("\n")
+
+	case "somepath":
+		if len(args) != 2 {
+			return fmt.Errorf("usage: somepath <from> <to>")
+		}
+		from, to := args[0], args[1]
+		if g[from] == nil {
+			return fmt.Errorf("no such 'from' node %q", from)
+		}
+		if g[to] == nil {
+			return fmt.Errorf("no such 'to' node %q", to)
+		}
+
+		seen := make(nodeset)
+		var visit func(path nodelist, label string) bool
+		visit = func(path nodelist, label string) bool {
+			if !seen[label] {
+				seen[label] = true
+				if label == to {
+					append(path, label).println("\n")
+					return true // unwind
+				}
+				for e := range g[label] {
+					if visit(append(path, label), e) {
+						return true
+					}
+				}
+			}
+			return false
+		}
+		if !visit(make(nodelist, 0, 100), from) {
+			return fmt.Errorf("no path from %q to %q", args[0], args[1])
+		}
+
+	case "allpaths":
+		if len(args) != 2 {
+			return fmt.Errorf("usage: allpaths <from> <to>")
+		}
+		from, to := args[0], args[1]
+		if g[from] == nil {
+			return fmt.Errorf("no such 'from' node %q", from)
+		}
+		if g[to] == nil {
+			return fmt.Errorf("no such 'to' node %q", to)
+		}
+
+		seen := make(nodeset) // value of seen[x] indicates whether x is on some path to 'to'
+		var visit func(label string) bool
+		visit = func(label string) bool {
+			reachesTo, ok := seen[label]
+			if !ok {
+				reachesTo = label == to
+
+				seen[label] = reachesTo
+				for e := range g[label] {
+					if visit(e) {
+						reachesTo = true
+					}
+				}
+				seen[label] = reachesTo
+			}
+			return reachesTo
+		}
+		if !visit(from) {
+			return fmt.Errorf("no path from %q to %q", from, to)
+		}
+		for label, reachesTo := range seen {
+			if !reachesTo {
+				delete(seen, label)
+			}
+		}
+		seen.sort().println("\n")
+
+	case "sccs":
+		if len(args) != 0 {
+			return fmt.Errorf("usage: sccs")
+		}
+		for _, scc := range g.sccs() {
+			scc.sort().println(" ")
+		}
+
+	case "scc":
+		if len(args) != 1 {
+			return fmt.Errorf("usage: scc <label>")
+		}
+		label := args[0]
+		if g[label] == nil {
+			return fmt.Errorf("no such node %q", label)
+		}
+		for _, scc := range g.sccs() {
+			if scc[label] {
+				scc.sort().println("\n")
+				break
+			}
+		}
+
+	default:
+		return fmt.Errorf("no such command %q", cmd)
+	}
+
+	return nil
+}
diff --git a/cmd/digraph/digraph_test.go b/cmd/digraph/digraph_test.go
new file mode 100644
index 00000000..28403d3e
--- /dev/null
+++ b/cmd/digraph/digraph_test.go
@@ -0,0 +1,62 @@
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"strings"
+	"testing"
+)
+
+func TestDigraph(t *testing.T) {
+	const g1 = `
+socks shoes
+shorts pants
+pants belt shoes
+shirt tie sweater
+sweater jacket
+hat
+`
+
+	const g2 = `
+a b c
+b d
+c d
+d c
+`
+
+	for _, test := range []struct {
+		input string
+		cmd   string
+		args  []string
+		want  string
+	}{
+		{g1, "nodes", nil, "belt\nhat\njacket\npants\nshirt\nshoes\nshorts\nsocks\nsweater\ntie\n"},
+		{g1, "reverse", []string{"jacket"}, "jacket\nshirt\nsweater\n"},
+		{g1, "forward", []string{"socks"}, "shoes\nsocks\n"},
+		{g1, "forward", []string{"socks", "sweater"}, "jacket\nshoes\nsocks\nsweater\n"},
+
+		{g2, "allpaths", []string{"a", "d"}, "a\nb\nc\nd\n"},
+
+		{g2, "sccs", nil, "a\nb\nc d\n"},
+		{g2, "scc", []string{"d"}, "c\nd\n"},
+		{g2, "succs", []string{"a"}, "b\nc\n"},
+		{g2, "preds", []string{"c"}, "a\nd\n"},
+		{g2, "preds", []string{"c", "d"}, "a\nb\nc\nd\n"},
+	} {
+		stdin = strings.NewReader(test.input)
+		stdout = new(bytes.Buffer)
+		if err := digraph(test.cmd, test.args); err != nil {
+			t.Error(err)
+			continue
+		}
+
+		got := stdout.(fmt.Stringer).String()
+		if got != test.want {
+			t.Errorf("digraph(%s, %s) = %q, want %q", test.cmd, test.args, got, test.want)
+		}
+	}
+
+	// TODO(adonovan):
+	// - test somepath (it's nondeterministic).
+	// - test errors
+}