diff --git a/src/pkg/encoding/gob/encoder_test.go b/src/pkg/encoding/gob/encoder_test.go
index 9a62cf9c2a..7911dad90d 100644
--- a/src/pkg/encoding/gob/encoder_test.go
+++ b/src/pkg/encoding/gob/encoder_test.go
@@ -685,3 +685,30 @@ func TestSliceIncompatibility(t *testing.T) {
 		t.Error("expected compatibility error")
 	}
 }
+
+// Mutually recursive slices of structs caused problems.
+type Bug3 struct {
+	Num      int
+	Children []*Bug3
+}
+
+func TestGobPtrSlices(t *testing.T) {
+	in := []*Bug3{
+		&Bug3{1, nil},
+		&Bug3{2, nil},
+	}
+	b := new(bytes.Buffer)
+	err := NewEncoder(b).Encode(&in)
+	if err != nil {
+		t.Fatal("encode:", err)
+	}
+
+	var out []*Bug3
+	err = NewDecoder(b).Decode(&out)
+	if err != nil {
+		t.Fatal("decode:", err)
+	}
+	if !reflect.DeepEqual(in, out) {
+		t.Fatal("got %v; wanted %v", out, in)
+	}
+}
diff --git a/src/pkg/encoding/gob/type.go b/src/pkg/encoding/gob/type.go
index 39006efdb2..0dd7a0a770 100644
--- a/src/pkg/encoding/gob/type.go
+++ b/src/pkg/encoding/gob/type.go
@@ -152,6 +152,10 @@ var idToType = make(map[typeId]gobType)
 var builtinIdToType map[typeId]gobType // set in init() after builtins are established
 
 func setTypeId(typ gobType) {
+	// When building recursive types, someone may get there before us.
+	if typ.id() != 0 {
+		return
+	}
 	nextId++
 	typ.setId(nextId)
 	idToType[nextId] = typ
@@ -346,6 +350,11 @@ func newSliceType(name string) *sliceType {
 func (s *sliceType) init(elem gobType) {
 	// Set our type id before evaluating the element's, in case it's our own.
 	setTypeId(s)
+	// See the comments about ids in newTypeObject. Only slices and
+	// structs have mutual recursion.
+	if elem.id() == 0 {
+		setTypeId(elem)
+	}
 	s.Elem = elem.id()
 }
 
@@ -503,6 +512,13 @@ func newTypeObject(name string, ut *userTypeInfo, rt reflect.Type) (gobType, err
 			if err != nil {
 				return nil, err
 			}
+			// Some mutually recursive types can cause us to be here while
+			// still defining the element. Fix the element type id here.
+			// We could do this more neatly by setting the id at the start of
+			// building every type, but that would break binary compatibility.
+			if gt.id() == 0 {
+				setTypeId(gt)
+			}
 			st.Field = append(st.Field, &fieldType{f.Name, gt.id()})
 		}
 		return st, nil