diff --git a/requests/sessions.py b/requests/sessions.py
index 0e430302..f032cdcf 100644
--- a/requests/sessions.py
+++ b/requests/sessions.py
@@ -134,7 +134,7 @@ class Session(object):
         :param headers: (optional) Dictionary of HTTP Headers to send with the :class:`Request`.
         :param cookies: (optional) Dict or CookieJar object to send with the :class:`Request`.
         :param files: (optional) Dictionary of 'filename': file-like-objects for multipart encoding upload.
-        :param auth: (optional) Auth tuple to enable Basic/Digest/Custom HTTP Auth.
+        :param auth: (optional) Auth tuple or callable to enable Basic/Digest/Custom HTTP Auth.
         :param timeout: (optional) Float describing the timeout of the request.
         :param allow_redirects: (optional) Boolean. Set to True by default.
         :param proxies: (optional) Dictionary mapping protocol to the URL of the proxy.