diff -r df79735b21c1 Lib/importlib/_bootstrap.py
--- a/Lib/importlib/_bootstrap.py	Fri Jul 05 01:39:49 2013 +0200
+++ b/Lib/importlib/_bootstrap.py	Fri Jul 05 23:50:19 2013 +0200
@@ -471,16 +471,16 @@
     """
     if len(bytecode_path) == 0:
         return None
-    rest, _, extension = bytecode_path.rparition('.')
-    if not rest or extension.lower()[-3:-1] != '.py':
+    rest, _, extension = bytecode_path.rpartition('.')
+    if not rest or extension.lower()[-3:-1] != 'py':
         return bytecode_path
 
     try:
         source_path = source_from_cache(bytecode_path)
     except (NotImplementedError, ValueError):
-        source_path = bytcode_path[-1:]
+        source_path = bytecode_path[:-1]
 
-    return source_path if _path_isfile(source_stats) else bytecode_path
+    return source_path if _path_isfile(source_path) else bytecode_path
 
 
 def _verbose_message(message, *args, verbosity=1):
diff -r df79735b21c1 Lib/test/test_import.py
--- a/Lib/test/test_import.py	Fri Jul 05 01:39:49 2013 +0200
+++ b/Lib/test/test_import.py	Fri Jul 05 23:50:19 2013 +0200
@@ -1,5 +1,6 @@
 # We import importlib *ASAP* in order to test #15386
 import importlib
+from importlib._bootstrap import _get_sourcefile
 import builtins
 import imp
 from test.test_importlib.import_ import util as importlib_util
@@ -11,6 +12,7 @@
 import stat
 import sys
 import unittest
+import unittest.mock as mock
 import textwrap
 import errno
 import shutil
@@ -864,6 +866,34 @@
         self.assertIs(imp.new_module, mod.new_module)
 
 
+@cpython_only
+class GetSourcefileTests(unittest.TestCase):
+
+    def test_get_sourcefile(self):
+        # Given a valid bytecode path, return the path to the corresponding
+        # source file if it exists. _bootstrap._path_isfile is mocked to avoid
+        # using actual .pyc and .py files for testing
+        with mock.patch('importlib._bootstrap._path_isfile') as _path_isfile:
+            _path_isfile.return_value = True;
+            path = TESTFN + '.pyc'
+            expect = TESTFN + '.py'
+            self.assertEqual(_get_sourcefile(path), expect)
+
+    def test_get_sourcefile_no_source(self):
+        # Given a valid bytecode path without a corresponding source path,
+        # return the original bytecode path
+        with mock.patch('importlib._bootstrap._path_isfile') as _path_isfile:
+            _path_isfile.return_value = False;
+            path = TESTFN + '.pyc'
+            self.assertEqual(_get_sourcefile(path), path)
+
+    def test_get_sourcefile_bad_ext(self):
+        # Given a path with an invalid bytecode extension, return the
+        # bytecode path passed as the argument
+        path = TESTFN + '.bad_ext'
+        self.assertEqual(_get_sourcefile(path), path)
+
+
 class ImportTracebackTests(unittest.TestCase):
 
     def setUp(self):
@@ -1028,7 +1058,7 @@
     run_unittest(ImportTests, PycacheTests, FilePermissionTests,
                  PycRewritingTests, PathsTests, RelativeImportTests,
                  OverridingImportBuiltinTests,
-                 ImportlibBootstrapTests,
+                 ImportlibBootstrapTests, GetSourcefileTests,
                  TestSymbolicallyLinkedPackage,
                  ImportTracebackTests)