diff --git a/mininet/node.py b/mininet/node.py
index a065533..566e3df 100644
--- a/mininet/node.py
+++ b/mininet/node.py
@@ -995,6 +995,12 @@ class Controller( Node ):
 
     def checkListening( self ):
         "Make sure no controllers are running on our port"
+        # Verify that Telnet is installed first:
+        out, err, returnCode = errRun( "which telnet" )
+        if 'telnet' not in out or returnCode != 0:
+            raise Exception( "Error running telnet to check for listening "
+                             "controllers; please check that it is "
+                             "installed." )
         listening = self.cmd( "echo A | telnet -e A %s %d" %
                               ( self.ip, self.port ) )
         if 'Unable' not in listening: