--- a/taskcluster/ci/source-test/python-tests.yml
+++ b/taskcluster/ci/source-test/python-tests.yml
@@ -94,17 +94,17 @@ mozharness:
         docker-image: {in-tree: "lint"}
         max-run-time: 1800
     run:
         using: run-task
         cache-dotcache: true
         command: >
             cd /home/worker/checkouts/gecko/testing/mozharness &&
             /usr/bin/pip2 install tox &&
-            /home/worker/.local/bin/tox -e py27-hg3.7
+            /home/worker/.local/bin/tox -e py27-hg4.1
     run-on-projects:
         - integration
         - release
     when:
         files-changed:
             - 'testing/mozharness/**'
 
 mozlint:

--- a/testing/mozharness/requirements.txt
+++ b/testing/mozharness/requirements.txt
@@ -3,17 +3,17 @@
 Cython==0.14.1
 Fabric==1.6.0
 coverage==3.6
 distribute==0.6.35
 dulwich==0.8.7
 hg-git==0.4.0
 logilab-astng==0.24.2
 logilab-common==0.59.0
-mercurial==3.7.3
+mercurial==4.1.2
 mock==1.0.1
 nose==1.2.1
 ordereddict==1.1
 paramiko==1.10.0
 pycrypto==2.6
 pyflakes==0.6.1
 pylint==0.27.0
 simplejson==2.1.1

--- a/testing/mozharness/tox.ini
+++ b/testing/mozharness/tox.ini
@@ -1,27 +1,27 @@
 [tox]
-envlist = py27-hg3.7
+envlist = py27-hg4.1
 
 [base]
 deps =
     coverage
     nose
     rednose
 
 [testenv]
 basepython = python2.7
 setenv =
     HGRCPATH = {toxinidir}/test/hgrc
 
 commands =
     coverage run --source configs,mozharness,scripts --branch {envbindir}/nosetests -v --with-xunit --rednose --force-color {posargs}
 
-[testenv:py27-hg3.7]
+[testenv:py27-hg4.1]
 deps =
     {[base]deps}
-    mercurial==3.7.3
+    mercurial==4.1.2
 
 [testenv:py27-coveralls]
 deps=
     python-coveralls==2.4.3
 commands=
     coveralls