diff --git a/bloaty/bloaty_merger_test.py b/bloaty/bloaty_merger_test.py
new file mode 100644
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+++ b/bloaty/bloaty_merger_test.py
@@ -0,0 +1,65 @@
+# Copyright 2021 Google Inc. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+import unittest
+
+from pyfakefs import fake_filesystem_unittest
+
+import bloaty_merger
+import file_sections_pb2
+
+
+class BloatyMergerTestCase(fake_filesystem_unittest.TestCase):
+  def setUp(self):
+    self.setUpPyfakefs()
+
+  def test_parse_csv(self):
+    csv_content = "sections,vmsize,filesize\nsection1,2,3\n"
+    self.fs.create_file("file1.bloaty.csv", contents=csv_content)
+    pb = bloaty_merger.parse_csv("file1.bloaty.csv")
+    self.assertEqual(pb.path, "file1")
+    self.assertEqual(len(pb.sections), 1)
+    s = pb.sections[0]
+    self.assertEqual(s.name, "section1")
+    self.assertEqual(s.vm_size, 2)
+    self.assertEqual(s.file_size, 3)
+
+  def test_missing_file(self):
+    with self.assertRaises(FileNotFoundError):
+      bloaty_merger.parse_csv("missing.bloaty.csv")
+
+  def test_malformed_csv(self):
+    csv_content = "header1,heaVder2,header3\n4,5,6\n"
+    self.fs.create_file("file1.bloaty.csv", contents=csv_content)
+    with self.assertRaises(KeyError):
+      bloaty_merger.parse_csv("file1.bloaty.csv")
+
+  def test_create_file_metrics(self):
+    file_list = "file1.bloaty.csv file2.bloaty.csv"
+    file1_content = "sections,vmsize,filesize\nsection1,2,3\nsection2,7,8"
+    file2_content = "sections,vmsize,filesize\nsection1,4,5\n"
+
+    self.fs.create_file("files.lst", contents=file_list)
+    self.fs.create_file("file1.bloaty.csv", contents=file1_content)
+    self.fs.create_file("file2.bloaty.csv", contents=file2_content)
+
+    bloaty_merger.create_file_size_metrics("files.lst", "output.pb")
+
+    metrics = file_sections_pb2.FileSizeMetrics()
+    with open("output.pb", "rb") as output:
+      metrics.ParseFromString(output.read())
+
+
+if __name__ == '__main__':
+  suite = unittest.TestLoader().loadTestsFromTestCase(BloatyMergerTestCase)
+  unittest.TextTestRunner(verbosity=2).run(suite)