diff --git a/tests/test_dir_util.py b/tests/test_dir_util.py
new file mode 100644
index 0000000..b8c3bcf
--- /dev/null
+++ b/tests/test_dir_util.py
@@ -0,0 +1,35 @@
+import unittest
+from unittest.mock import MagicMock, patch
+
+from cellxgene_gateway.dir_util import ensure_dir_exists, make_annotations, make_h5ad
+
+
+class TestMakeH5ad(unittest.TestCase):
+    def test_GIVEN_annotation_dir_THEN_returns_h5ad(self):
+        self.assertEqual(make_h5ad("pbmc_annotations"), "pbmc.h5ad")
+
+
+class TestMakeAnnotations(unittest.TestCase):
+    def test_GIVEN_h5ad_THEN_returns_annotations(self):
+        self.assertEqual(make_annotations("pbmc.h5ad"), "pbmc_annotations")
+
+
+class TestMakeAnnotations(unittest.TestCase):
+    def test_GIVEN_h5ad_THEN_returns_annotations(self):
+        self.assertEqual(make_annotations("pbmc.h5ad"), "pbmc_annotations")
+
+
+class TestEnsureDirExists(unittest.TestCase):
+    @patch("os.path.exists")
+    @patch("os.makedirs")
+    def test_GIVEN_existing_THEN_does_not_call_makedir(self, makedirsMock, existsMock):
+        existsMock.return_value = True
+        ensure_dir_exists("/foo")
+        makedirsMock.assert_not_called()
+
+    @patch("os.path.exists")
+    @patch("os.makedirs")
+    def test_GIVEN_not_existing_THEN_calls_makedir(self, makedirsMock, existsMock):
+        existsMock.return_value = False
+        ensure_dir_exists("/foo")
+        makedirsMock.assert_called_once_with("/foo")