diff --git a/main_test.go b/main_test.go
index 4bed922..292a20f 100644
--- a/main_test.go
+++ b/main_test.go
@@ -2,6 +2,7 @@ package main
 
 import (
 	"gopkg.in/check.v1"
+	"path/filepath"
 )
 
 func (s *Suite) Test_gobco_parseCommandLine(c *check.C) {
@@ -60,3 +61,16 @@ func (s *Suite) Test_gobco_parseCommandLine__two_packages(c *check.C) {
 		"src/github.com/rillig/gobco/pkg1",
 		"src/github.com/rillig/gobco/pkg2"})
 }
+
+func (s *Suite) Test_gobco_instrument__gobco_files(c *check.C) {
+	var g gobco
+	g.parseCommandLine([]string{"gobco", "sample"})
+	g.prepareTmpEnv()
+	g.instrument()
+
+	c.Check(listRegularFiles(filepath.Join(g.tmpdir, g.tmpItems[0])), check.DeepEquals, []string{
+		"foo.go",
+		"foo_test.go",
+		"gobco.go",
+		"gobco_test.go"})
+}
diff --git a/util_test.go b/util_test.go
new file mode 100644
index 0000000..f3ee583
--- /dev/null
+++ b/util_test.go
@@ -0,0 +1,26 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+func listRegularFiles(basedir string) []string {
+	var files []string
+
+	err := filepath.Walk(basedir, func(path string, info os.FileInfo, err error) error {
+		if err == nil && info.Mode().IsRegular() {
+			rel := strings.TrimPrefix(path, basedir)
+			slashed := filepath.ToSlash(rel)
+			files = append(files, strings.TrimPrefix(slashed, "/"))
+		}
+		return err
+	})
+
+	if err != nil {
+		panic(err)
+	}
+
+	return files
+}