diff --git a/Lib/mailbox.py b/Lib/mailbox.py
index b72128b4db4..eab03afb7c9 100755
--- a/Lib/mailbox.py
+++ b/Lib/mailbox.py
@@ -578,7 +578,7 @@ class _singlefileMailbox(Mailbox):
         self._toc = new_toc
         self._pending = False
         if self._locked:
-            _lock_file(new_file, dotlock=False)
+            _lock_file(self._file, dotlock=False)
 
     def _pre_mailbox_hook(self, f):
         """Called before writing the mailbox to file f."""
diff --git a/Lib/test/test_mailbox.py b/Lib/test/test_mailbox.py
index 6cdc441ee2e..40cf1929056 100644
--- a/Lib/test/test_mailbox.py
+++ b/Lib/test/test_mailbox.py
@@ -747,6 +747,22 @@ class _TestMboxMMDF(TestMailbox):
         self._box.lock()
         self._box.unlock()
 
+    def test_relock(self):
+        # Test case for bug #1575506: the mailbox class was locking the
+        # wrong file object in its flush() method.
+        msg = "Subject: sub\n\nbody\n"
+        key1 = self._box.add(msg)
+        self._box.flush()
+        self._box.close()
+        
+        self._box = self._factory(self._path)
+        self._box.lock()
+        key2 = self._box.add(msg)
+        self._box.flush()
+        self.assert_(self._box._locked)
+        self._box.close()
+        
+        
 
 class TestMbox(_TestMboxMMDF):
 
diff --git a/Misc/NEWS b/Misc/NEWS
index 13f52a78e7b..64fa7bf826e 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -105,6 +105,9 @@ Library
 - Bug #1565661: in webbrowser, split() the command for the default
   GNOME browser in case it is a command with args.
 
+- Bug #1575506: Single-file mailboxes didn't re-lock properly in
+  their flush() method.
+
 - Bug #1576241: fix functools.wraps() to work on built-in functions.
 
 - Fix a bug in traceback.format_exception_only() that led to an error