--- a/tests/run-tests.py	Wed Aug 02 21:01:38 2017 -0700
+++ b/tests/run-tests.py	Wed Aug 02 21:24:29 2017 -0700
@@ -406,6 +406,10 @@
                       metavar="known_good_rev",
                       help=("Automatically bisect any failures using this "
                             "revision as a known-good revision."))
+    parser.add_option('--bisect-repo', type="string",
+                      metavar='bisect_repo',
+                      help=("Path of a repo to bisect. Use together with "
+                            "--known-good-rev"))
 
     for option, (envvar, default) in defaults.items():
         defaults[option] = type(default)(os.environ.get(envvar, default))
@@ -458,6 +462,9 @@
         sys.stderr.write('warning: --color=always ignored because '
                          'pygments is not installed\n')
 
+    if options.bisect_repo and not options.known_good_rev:
+        parser.error("--bisect-repo cannot be used without --known-good-rev")
+
     global useipv6
     if options.ipv6:
         useipv6 = checksocketfamily('AF_INET6')
@@ -2071,6 +2078,9 @@
 
             if failed and self._runner.options.known_good_rev:
                 bisectcmd = ['hg', 'bisect']
+                bisectrepo = self._runner.options.bisect_repo
+                if bisectrepo:
+                    bisectcmd.extend(['-R', os.path.abspath(bisectrepo)])
                 def nooutput(args):
                     p = subprocess.Popen(args, stderr=subprocess.STDOUT,
                                          stdout=subprocess.PIPE)

--- a/tests/test-run-tests.t	Wed Aug 02 21:01:38 2017 -0700
+++ b/tests/test-run-tests.t	Wed Aug 02 21:24:29 2017 -0700
@@ -1285,6 +1285,58 @@
 
   $ cd ..
 
+support bisecting a separate repo
+
+  $ hg init bisect-dependent
+  $ cd bisect-dependent
+  $ cat > test-bisect-dependent.t <<EOF
+  >   $ tail -1 \$TESTDIR/../bisect/test-bisect.t
+  >     pass
+  > EOF
+  $ hg commit -Am dependent test-bisect-dependent.t
+
+  $ rt --known-good-rev=0 test-bisect-dependent.t
+  
+  --- $TESTTMP/anothertests/bisect-dependent/test-bisect-dependent.t
+  +++ $TESTTMP/anothertests/bisect-dependent/test-bisect-dependent.t.err
+  @@ -1,2 +1,2 @@
+     $ tail -1 $TESTDIR/../bisect/test-bisect.t
+  -    pass
+  +    fail
+  
+  ERROR: test-bisect-dependent.t output changed
+  !
+  Failed test-bisect-dependent.t: output changed
+  Failed to identify failure point for test-bisect-dependent.t
+  # Ran 1 tests, 0 skipped, 1 failed.
+  python hash seed: * (glob)
+  [1]
+
+  $ rt --bisect-repo=../test-bisect test-bisect-dependent.t
+  Usage: run-tests.py [options] [tests]
+  
+  run-tests.py: error: --bisect-repo cannot be used without --known-good-rev
+  [2]
+
+  $ rt --known-good-rev=0 --bisect-repo=../bisect test-bisect-dependent.t
+  
+  --- $TESTTMP/anothertests/bisect-dependent/test-bisect-dependent.t
+  +++ $TESTTMP/anothertests/bisect-dependent/test-bisect-dependent.t.err
+  @@ -1,2 +1,2 @@
+     $ tail -1 $TESTDIR/../bisect/test-bisect.t
+  -    pass
+  +    fail
+  
+  ERROR: test-bisect-dependent.t output changed
+  !
+  Failed test-bisect-dependent.t: output changed
+  test-bisect-dependent.t broken by 72cbf122d116 (bad)
+  # Ran 1 tests, 0 skipped, 1 failed.
+  python hash seed: * (glob)
+  [1]
+
+  $ cd ..
+
 Test a broken #if statement doesn't break run-tests threading.
 ==============================================================
   $ mkdir broken