diff --git a/tests/test__default.py b/tests/test__default.py
index 137fdcd..747bfa4 100644
--- a/tests/test__default.py
+++ b/tests/test__default.py
@@ -19,6 +19,7 @@
 import pytest
 
 from google.auth import _default
+from google.auth import app_engine
 from google.auth import compute_engine
 from google.auth import environment_vars
 from google.auth import exceptions
@@ -188,7 +189,25 @@
     assert project_id is None
 
 
-def test__get_gae_credentials():
+@pytest.fixture
+def app_identity_mock(monkeypatch):
+    """Mocks the app_identity module for google.auth.app_engine."""
+    app_identity_mock = mock.Mock()
+    monkeypatch.setattr(
+        app_engine, 'app_identity', app_identity_mock)
+    yield app_identity_mock
+
+
+def test__get_gae_credentials(app_identity_mock):
+    app_identity_mock.get_application_id.return_value = mock.sentinel.project
+
+    credentials, project_id = _default._get_gae_credentials()
+
+    assert isinstance(credentials, app_engine.Credentials)
+    assert project_id == mock.sentinel.project
+
+
+def test__get_gae_credentials_no_apis():
     assert _default._get_gae_credentials() == (None, None)
 
 
diff --git a/tests/test_app_engine.py b/tests/test_app_engine.py
index a9844b0..e1189ed 100644
--- a/tests/test_app_engine.py
+++ b/tests/test_app_engine.py
@@ -29,6 +29,18 @@
     yield app_identity_mock
 
 
+def test_get_project_id(app_identity_mock):
+    app_identity_mock.get_application_id.return_value = mock.sentinel.project
+    assert app_engine.get_project_id() == mock.sentinel.project
+
+
+def test_get_project_id_missing_apis():
+    with pytest.raises(EnvironmentError) as excinfo:
+        assert app_engine.get_project_id()
+
+    assert excinfo.match(r'App Engine APIs are not available')
+
+
 class TestCredentials(object):
     def test_missing_apis(self):
         with pytest.raises(EnvironmentError) as excinfo: