diff --git a/src/wormhole/scripts/cli_args.py b/src/wormhole/scripts/cli_args.py
index 369ab20..fd8e11f 100644
--- a/src/wormhole/scripts/cli_args.py
+++ b/src/wormhole/scripts/cli_args.py
@@ -27,6 +27,8 @@ g.add_argument("--hide-progress", action="store_true",
                help="supress progress-bar display")
 g.add_argument("--dump-timing", type=type(u""), # TODO: hide from --help output
                metavar="FILE", help="(debug) write timing data to file")
+g.add_argument("--twisted", action="store_true",
+               help="use Twisted-based implementations, for testing")
 parser.set_defaults(timing=None)
 subparsers = parser.add_subparsers(title="subcommands",
                                    dest="subcommand")
diff --git a/src/wormhole/scripts/runner.py b/src/wormhole/scripts/runner.py
index d68b353..f89f01e 100644
--- a/src/wormhole/scripts/runner.py
+++ b/src/wormhole/scripts/runner.py
@@ -20,12 +20,20 @@ def dispatch(args):
     if args.func == "usage/tail":
         from ..servers import cmd_usage
         return cmd_usage.tail_usage(args)
+
     if args.func == "send/send":
+        if args.twisted:
+            from . import cmd_send_twisted
+            return cmd_send_twisted.send_twisted_sync(args)
         from . import cmd_send_blocking
         return cmd_send_blocking.send_blocking(args)
     if args.func == "receive/receive":
+        if args.twisted:
+            from . import cmd_receive_twisted
+            return cmd_receive_twisted.receive_twisted_sync(args)
         from . import cmd_receive_blocking
         return cmd_receive_blocking.receive_blocking(args)
+
     raise ValueError("unknown args.func %s" % args.func)
 
 def run(args, cwd, stdout, stderr, executable=None):