diff --git a/mercurial/bundle2.py b/mercurial/bundle2.py
--- a/mercurial/bundle2.py
+++ b/mercurial/bundle2.py
@@ -97,7 +97,8 @@ class bundle20(object):
 
     This object does not support payload part yet."""
 
-    def __init__(self):
+    def __init__(self, ui):
+        self.ui = ui
         self._params = []
         self._parts = []
 
@@ -110,8 +111,10 @@ class bundle20(object):
         self._params.append((name, value))
 
     def getchunks(self):
+        self.ui.debug('start emission of %s stream\n' % _magicstring)
         yield _magicstring
         param = self._paramchunk()
+        self.ui.debug('bundle parameter: %s\n' % param)
         yield _pack(_fstreamparamsize, len(param))
         if param:
             yield param
@@ -119,6 +122,7 @@ class bundle20(object):
         # no support for parts
         # to be obviously fixed soon.
         assert not self._parts
+        self.ui.debug('end of bundle\n')
         yield '\0\0'
 
     def _paramchunk(self):
diff --git a/tests/test-bundle2.t b/tests/test-bundle2.t
--- a/tests/test-bundle2.t
+++ b/tests/test-bundle2.t
@@ -20,7 +20,7 @@ Create an extension to test bundle2 API
   >          '[OUTPUTFILE]')
   > def cmdbundle2(ui, repo, path=None, **opts):
   >     """write a bundle2 container on standard ouput"""
-  >     bundler = bundle2.bundle20()
+  >     bundler = bundle2.bundle20(ui)
   >     for p in opts['param']:
   >         p = p.split('=', 1)
   >         try:
@@ -161,9 +161,11 @@ Test unbundling
 
 Test debug output
 ---------------------------------------------------
-(no debug output yet)
 
   $ hg bundle2 --debug --param 'e|! 7/=babar%#==tutu' --param simple ../out.hg2
+  start emission of HG20 stream
+  bundle parameter: e%7C%21%207/=babar%25%23%3D%3Dtutu simple
+  end of bundle
 
 file content is ok