diff --git a/Lib/unittest/mock.py b/Lib/unittest/mock.py
index 9d9b3e2..979f657 100644
--- a/Lib/unittest/mock.py
+++ b/Lib/unittest/mock.py
@@ -2265,7 +2265,14 @@ def _iterate_read_data(read_data):
     # Helper for mock_open:
     # Retrieve lines from read_data via a generator so that separate calls to
     # readline, read, and readlines are properly interleaved
-    data_as_list = ['{}\n'.format(l) for l in read_data.split('\n')]
+
+    if type(read_data) == bytes:
+        data_as_list = [l + b'\n' for l in
+                read_data.split(b'\n')]
+    else:
+        data_as_list = [l + '\n' for l in
+                read_data.split('\n')]
+
 
     if data_as_list[-1] == '\n':
         # If the last line ended in a newline, the list comprehension will have an
@@ -2300,7 +2307,7 @@ def mock_open(mock=None, read_data=''):
     def _read_side_effect(*args, **kwargs):
         if handle.read.return_value is not None:
             return handle.read.return_value
-        return ''.join(_data)
+        return type(read_data)().join(_data)
 
     def _readline_side_effect():
         if handle.readline.return_value is not None:
diff --git a/Lib/unittest/test/testmock/testwith.py b/Lib/unittest/test/testmock/testwith.py
index f54e051..b9e316f 100644
--- a/Lib/unittest/test/testmock/testwith.py
+++ b/Lib/unittest/test/testmock/testwith.py
@@ -210,6 +210,20 @@ class TestMockOpen(unittest.TestCase):
         self.assertEqual(result, ['foo\n', 'bar\n', 'baz'])
 
 
+    def test_read_bytes(self):
+        m = mock_open(read_data=b'\xc6')
+        with patch('%s.open' % __name__, m, create=True) :
+            with open('abc', 'rb') as f:
+                self.assertEqual(f.read(), b'\xc6')
+
+
+    def test_readlines_bytes(self):
+        m = mock_open(read_data=b'abc\ndef\nghi')
+        with patch('%s.open' % __name__, m, create=True) :
+            with open('abc', 'rb') as f:
+                self.assertEqual(f.readlines(), [b'abc\n', b'def\n', b'ghi'])
+
+
     def test_mock_open_read_with_argument(self):
         # At one point calling read with an argument was broken
         # for mocks returned by mock_open