diff --git a/var/spack/packages/python/package.py b/var/spack/packages/python/package.py
index 8a6d574d9b2..23b528b0897 100644
--- a/var/spack/packages/python/package.py
+++ b/var/spack/packages/python/package.py
@@ -66,6 +66,11 @@ def setup_extension_environment(self, module, spec, ext_spec):
         # Make the site packages directory if it does not exist already.
         mkdirp(module.site_packages_dir)
 
+        # Add dependent packages' site-packages directory to PYTHONPATH
+        for d in ext_spec.traverse():
+            if d.package.extends(self.spec):
+                os.environ['PYTHONPATH'] += ':' + os.path.join(d.prefix, self.site_packages_dir)
+
 
     # ========================================================================
     # Handle specifics of activating and deactivating python modules.