diff --git a/google/appengine.go b/google/appengine.go
index 4243f4c..50d918b 100644
--- a/google/appengine.go
+++ b/google/appengine.go
@@ -14,8 +14,8 @@
 	"golang.org/x/oauth2"
 )
 
-// Set at init time by appenginevm_hook.go. If true, we are on App Engine Managed VMs.
-var appengineVM bool
+// appengineFlex is set at init time by appengineflex_hook.go. If true, we are on App Engine Flex.
+var appengineFlex bool
 
 // Set at init time by appengine_hook.go. If nil, we're not on App Engine.
 var appengineTokenFunc func(c context.Context, scopes ...string) (token string, expiry time.Time, err error)
diff --git a/google/appengine_hook.go b/google/appengine_hook.go
index 6f66411..56669ea 100644
--- a/google/appengine_hook.go
+++ b/google/appengine_hook.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build appengine
+// +build appengine appenginevm
 
 package google
 
diff --git a/google/appengineflex_hook.go b/google/appengineflex_hook.go
new file mode 100644
index 0000000..5d0231a
--- /dev/null
+++ b/google/appengineflex_hook.go
@@ -0,0 +1,11 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build appenginevm
+
+package google
+
+func init() {
+	appengineFlex = true // Flex doesn't support appengine.AccessToken; depend on metadata server.
+}
diff --git a/google/appenginevm_hook.go b/google/appenginevm_hook.go
deleted file mode 100644
index 1074780..0000000
--- a/google/appenginevm_hook.go
+++ /dev/null
@@ -1,15 +0,0 @@
-// Copyright 2015 The oauth2 Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// +build appenginevm
-
-package google
-
-import "google.golang.org/appengine"
-
-func init() {
-	appengineVM = true
-	appengineTokenFunc = appengine.AccessToken
-	appengineAppIDFunc = appengine.AppID
-}
diff --git a/google/default.go b/google/default.go
index b45e796..004ed4e 100644
--- a/google/default.go
+++ b/google/default.go
@@ -81,7 +81,7 @@
 	}
 
 	// Third, if we're on Google App Engine use those credentials.
-	if appengineTokenFunc != nil && !appengineVM {
+	if appengineTokenFunc != nil && !appengineFlex {
 		return &DefaultCredentials{
 			ProjectID:   appengineAppIDFunc(ctx),
 			TokenSource: AppEngineTokenSource(ctx, scope...),