diff --git a/mininet.py b/mininet.py
index bba518ea3f2f2ec279add8af902bb57d98b919d9..d58e88f44b2b4332de29e94e54907c84fab4d27a 100755
--- a/mininet.py
+++ b/mininet.py
@@ -796,6 +796,10 @@ def init():
       # Note: this script must be run as root 
       # Perhaps we should do so automatically!
       print "*** Mininet must run as root."; exit( 1 )
+   # If which produces no output, then netns is not in the path.
+   # May want to loosen this to handle netns in the current dir.
+   if not quietRun(['which', 'netns']):
+       raise Exception("Could not find netns; see INSTALL")
    fixLimits()
 
 if __name__ == '__main__':