diff --git a/libmproxy/cmdline.py b/libmproxy/cmdline.py
index b892f1fd8..bf5add336 100644
--- a/libmproxy/cmdline.py
+++ b/libmproxy/cmdline.py
@@ -179,7 +179,7 @@ def get_common_options(options):
         stickyauth=stickyauth,
         stream_large_bodies=stream_large_bodies,
         showhost=options.showhost,
-        wfile=options.wfile,
+        outfile=options.outfile,
         verbosity=options.verbose,
         nopop=options.nopop,
         replay_ignore_content = options.replay_ignore_content,
@@ -249,11 +249,17 @@ def common_options(parser):
         action="store_const", dest="verbose", default=1, const=2,
         help="Increase event log verbosity."
     )
-    parser.add_argument(
+    outfile = parser.add_mutually_exclusive_group()
+    outfile.add_argument(
         "-w", "--wfile",
-        action="store", dest="wfile", default=None,
+        action="store", dest="outfile", type=lambda f: (f, "wb"),
         help="Write flows to file."
     )
+    outfile.add_argument(
+        "-a", "--afile",
+        action="store", dest="outfile", type=lambda f: (f, "ab"),
+        help="Append flows to file."
+    )
     parser.add_argument(
         "-z", "--anticomp",
         action="store_true", dest="anticomp", default=False,
@@ -371,7 +377,7 @@ def common_options(parser):
 
     group = parser.add_argument_group("Onboarding App")
     group.add_argument(
-        "-a", "--noapp",
+        "--noapp",
         action="store_false", dest="app", default=True,
         help="Disable the mitmproxy onboarding app."
     )
diff --git a/libmproxy/dump.py b/libmproxy/dump.py
index 0d9432c9c..8f2607458 100644
--- a/libmproxy/dump.py
+++ b/libmproxy/dump.py
@@ -36,7 +36,7 @@ class Options(object):
         "stickyauth",
         "stream_large_bodies",
         "verbosity",
-        "wfile",
+        "outfile",
         "replay_ignore_content",
         "replay_ignore_params",
     ]
@@ -92,10 +92,10 @@ class DumpMaster(flow.FlowMaster):
         if options.stickyauth:
             self.set_stickyauth(options.stickyauth)
 
-        if options.wfile:
-            path = os.path.expanduser(options.wfile)
+        if options.outfile:
+            path = os.path.expanduser(options.outfile[0])
             try:
-                f = file(path, "wb")
+                f = file(path, options.outfile[1])
                 self.start_stream(f, self.filt)
             except IOError, v:
                 raise DumpError(v.strerror)
diff --git a/test/test_dump.py b/test/test_dump.py
index e9cb4d331..aa91d2627 100644
--- a/test/test_dump.py
+++ b/test/test_dump.py
@@ -143,9 +143,16 @@ class TestDumpMaster:
     def test_write(self):
         with tutils.tmpdir() as d:
             p = os.path.join(d, "a")
-            self._dummy_cycle(1, None, "", wfile=p, verbosity=0)
+            self._dummy_cycle(1, None, "", outfile=(p,"wb"), verbosity=0)
             assert len(list(flow.FlowReader(open(p,"rb")).stream())) == 1
 
+    def test_write_append(self):
+        with tutils.tmpdir() as d:
+            p = os.path.join(d, "a.append")
+            self._dummy_cycle(1, None, "", outfile=(p,"wb"), verbosity=0)
+            self._dummy_cycle(1, None, "", outfile=(p,"ab"), verbosity=0)
+            assert len(list(flow.FlowReader(open(p,"rb")).stream())) == 2
+
     def test_write_err(self):
         tutils.raises(
             dump.DumpError,
@@ -153,7 +160,7 @@ class TestDumpMaster:
             1,
             None,
             "",
-            wfile = "nonexistentdir/foo"
+            outfile = ("nonexistentdir/foo", "wb")
         )
 
     def test_script(self):