diff --git a/swh/loader/mercurial/tests/conftest.py b/swh/loader/mercurial/tests/conftest.py
index ee9c3f8..132fc24 100644
--- a/swh/loader/mercurial/tests/conftest.py
+++ b/swh/loader/mercurial/tests/conftest.py
@@ -1,39 +1,49 @@
 # Copyright (C) 2019-2021  The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
+import os
 from typing import Any, Dict
 
 import pytest
 
 
 @pytest.fixture
 def swh_storage_backend_config(swh_storage_backend_config):
     """Basic pg storage configuration with no journal collaborator
     (to avoid pulling optional dependency on clients of this fixture)
 
     """
     return {
         "cls": "filter",
         "storage": {
             "cls": "buffer",
             "min_batch_size": {
                 "content": 10,
                 "content_bytes": 100 * 1024 * 1024,
                 "directory": 10,
                 "revision": 10,
                 "release": 10,
             },
             "storage": swh_storage_backend_config,
         },
     }
 
 
 @pytest.fixture
 def swh_loader_config(swh_storage_backend_config, tmp_path) -> Dict[str, Any]:
     return {
         "storage": swh_storage_backend_config,
         "max_content_size": 104857600,
         "temp_directory": str(tmp_path),
     }
+
+
+@pytest.fixture(autouse=True, scope="session")
+def swh_mercurial_set_plain():
+    """Mercurial is customizable by users, so we use built-in environment
+    variables to turn off all customization in tests.
+    """
+    os.environ["HGPLAIN"] = ""
+    os.environ["HGRCPATH"] = ""