diff --git a/tox.ini b/tox.ini
index b8c6aca33..3d8f0d467 100644
--- a/tox.ini
+++ b/tox.ini
@@ -14,6 +14,13 @@ skipsdist = True
 [testenv]
 basepython=python2.7
 passenv = TAHOE_LAFS_* PIP_* SUBUNITREPORTER_* USERPROFILE HOMEDRIVE HOMEPATH
+setenv =
+       # With pip >= 10 the existence of pyproject.toml (which we are required
+       # to have to configure towncrier) triggers a "build isolation" mode
+       # which prevents anything from working.  Un-trigger this pip behavior
+       # so that packages pre-installed in a virtualenv are available.
+       PIP_NO_BUILD_ISOLATION=1
+
 # Get "certifi" to avoid bug #2913. Basically if a `setup_requires=...` causes
 # a package to be installed (with setuptools) then it'll fail on certain
 # platforms (travis's OX-X 10.12, Slackware 14.2) because PyPI's TLS