diff --git a/tox.ini b/tox.ini
--- a/tox.ini
+++ b/tox.ini
@@ -6,7 +6,9 @@
   .[testing]
   pytest-cov
 commands =
-  pytest --cov=swh --cov-branch {posargs}
+  pytest --cov={envsitepackagesdir}/swh/loader/mercurial \
+         {envsitepackagesdir}/swh/loader/mercurial \
+         --cov-branch {posargs}
 
 [testenv:flake8]
 skip_install = true