diff --git a/pre_commit/clientlib/validate_manifest.py b/pre_commit/clientlib/validate_manifest.py
index 035114e3..8fe71480 100644
--- a/pre_commit/clientlib/validate_manifest.py
+++ b/pre_commit/clientlib/validate_manifest.py
@@ -8,6 +8,7 @@ import os.path
 import yaml
 
 import pre_commit.constants as C
+from pre_commit import git
 
 
 class InvalidManifestError(ValueError): pass
@@ -69,8 +70,7 @@ def run(argv):
     args = parser.parse_args(argv)
 
     if args.filename is None:
-        # TODO: filename = git.get_root() + C.MANIFEST_FILE
-        raise NotImplementedError
+        filename = os.path.join(git.get_root(), C.MANIFEST_FILE)
     else:
         filename = args.filename
 
diff --git a/tests/clientlib/validate_manifest_test.py b/tests/clientlib/validate_manifest_test.py
index 7996525d..cef68d70 100644
--- a/tests/clientlib/validate_manifest_test.py
+++ b/tests/clientlib/validate_manifest_test.py
@@ -3,7 +3,9 @@ import __builtin__
 import jsonschema
 import pytest
 import mock
+from plumbum import local
 
+import pre_commit.constants as C
 from pre_commit.clientlib.validate_manifest import check_is_valid_manifest
 from pre_commit.clientlib.validate_manifest import InvalidManifestError
 from pre_commit.clientlib.validate_manifest import run
@@ -48,6 +50,18 @@ def test_returns_0_for_valid_manifest():
     assert ret == 0
 
 
+def test_uses_default_manifest_file_at_root_of_git(empty_git_dir):
+    local.path(C.MANIFEST_FILE).write("""
+hooks:
+    -
+        id: foo
+        name: Foo
+        entry: foo
+    """)
+    ret = run([])
+    assert ret == 0
+
+
 @pytest.mark.parametrize(('manifest', 'expected_exception_type'), (
     (
         """
diff --git a/tests/conftest.py b/tests/conftest.py
new file mode 100644
index 00000000..68613e1a
--- /dev/null
+++ b/tests/conftest.py
@@ -0,0 +1,10 @@
+
+import pytest
+from plumbum import local
+
+
+@pytest.yield_fixture
+def empty_git_dir(tmpdir):
+    with local.cwd(tmpdir.strpath):
+        local['git']['init']()
+        yield tmpdir.strpath
\ No newline at end of file
diff --git a/tests/git_test.py b/tests/git_test.py
index c566b1f3..cb0331e4 100644
--- a/tests/git_test.py
+++ b/tests/git_test.py
@@ -1,36 +1,17 @@
 
-import contextlib
 import os
-import pytest
 
 from plumbum import local
 from pre_commit import git
 
 
-
-@contextlib.contextmanager
-def in_dir(dir):
-    old_path = local.cwd.getpath()
-    local.cwd.chdir(dir)
-    try:
-        yield
-    finally:
-        local.cwd.chdir(old_path)
-
-@pytest.yield_fixture
-def empty_git_dir(tmpdir):
-    with in_dir(tmpdir.strpath):
-        local['git']['init']()
-        yield tmpdir.strpath
-
-
 def test_get_root(empty_git_dir):
     assert git.get_root() == empty_git_dir
 
     foo = local.path('foo')
     foo.mkdir()
 
-    with in_dir(foo):
+    with local.cwd(foo):
         assert git.get_root() == empty_git_dir