diff --git a/Lib/os.py b/Lib/os.py
index 2fdf668..19a045a 100644
--- a/Lib/os.py
+++ b/Lib/os.py
@@ -27,7 +27,7 @@
 _names = sys.builtin_module_names
 
 # Note:  more names are added to __all__ later.
-__all__ = ["altsep", "curdir", "pardir", "sep", "pathsep", "linesep",
+__all__ = ["altsep", "curdir", "pardir", "sep", "extsep", "pathsep", "linesep",
            "defpath", "name", "path", "devnull",
            "SEEK_SET", "SEEK_CUR", "SEEK_END"]