diff --git a/Lib/unittest/mock.py b/Lib/unittest/mock.py
index 9d9b3e2..ad8ca6a 100644
--- a/Lib/unittest/mock.py
+++ b/Lib/unittest/mock.py
@@ -2265,7 +2265,18 @@ 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 it's a bytes object, temporarily decode it, because format() is only
+    # available on str
+    if type(read_data) == bytes:
+        read_data = read_data.decode()
+
+        data_as_list = ['{}\n'.format(l).encode() for l in
+                read_data.split('\n')]
+    else:
+        data_as_list = ['{}\n'.format(l) 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 +2311,7 @@ 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/testmock.py b/Lib/unittest/test/testmock/testmock.py
index 3a104cb..b3e0f90 100644
--- a/Lib/unittest/test/testmock/testmock.py
+++ b/Lib/unittest/test/testmock/testmock.py
@@ -8,7 +8,7 @@ from unittest.mock import (
     call, DEFAULT, patch, sentinel,
     MagicMock, Mock, NonCallableMock,
     NonCallableMagicMock, _CallList,
-    create_autospec
+    create_autospec, mock_open
 )
 
 
@@ -1444,5 +1444,18 @@ class MockTest(unittest.TestCase):
             mock.foo
 
 
+    def test_mock_open_read_bytes(self):
+        m = mock_open(read_data= b'abc')
+        with patch('%s.open' % __name__, m, create=True) :
+            with open('abc', 'rb') as f:
+                self.assertEqual(f.read(), b'abc')
+
+
+    def test_mock_open_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'])
+
 if __name__ == '__main__':
     unittest.main()
diff --git a/Lib/unittest/test/testmock/testwith.py b/Lib/unittest/test/testmock/testwith.py
index f54e051..dd97968 100644
--- a/Lib/unittest/test/testmock/testwith.py
+++ b/Lib/unittest/test/testmock/testwith.py
@@ -139,6 +139,8 @@ class TestMockOpen(unittest.TestCase):
         mock.assert_called_once_with('foo')
 
 
+
+
     def test_mock_open_context_manager(self):
         mock = mock_open()
         handle = mock.return_value
@@ -210,6 +212,20 @@ class TestMockOpen(unittest.TestCase):
         self.assertEqual(result, ['foo\n', 'bar\n', 'baz'])
 
 
+    def test_read_bytes(self):
+        m = mock_open(read_data= b'abc')
+        with patch('%s.open' % __name__, m, create=True) :
+            with open('abc', 'rb') as f:
+                self.assertEqual(f.read(), b'abc')
+
+
+    def testreadlines_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
@@ -255,5 +271,6 @@ class TestMockOpen(unittest.TestCase):
         self.assertEqual(handle.readline(), 'bar')
 
 
+
 if __name__ == '__main__':
     unittest.main()