diff --git a/python/setup.py b/python/setup.py
index 1e71a70..2a5513b 100755
--- a/python/setup.py
+++ b/python/setup.py
@@ -183,7 +183,7 @@
     os.environ['PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION'] = 'cpp'
 
   # Keep this list of dependencies in sync with tox.ini.
-  install_requires = ['six', 'setuptools']
+  install_requires = ['six>=1.9', 'setuptools']
   if sys.version_info <= (2,7):
     install_requires.append('ordereddict')
     install_requires.append('unittest2')
diff --git a/python/tox.ini b/python/tox.ini
index 3ca3934..301335b 100644
--- a/python/tox.ini
+++ b/python/tox.ini
@@ -21,6 +21,6 @@
 # cpp: python setup.py -q test_conformance --cpp_implementation
 deps =
     # Keep this list of dependencies in sync with setup.py.
-    six
+    six>=1.9
     py26: ordereddict
     py26: unittest2