diff --git a/CHANGES.rst b/CHANGES.rst
index cb909c7..63ce8dc 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -5,6 +5,8 @@
 6.1 (unreleased)
 ================
 
+- Respect ``PURE_PYTHON`` environment variable set to ``0``.
+
 
 6.0 (2024-09-17)
 ================
diff --git a/src/zope/proxy/__init__.py b/src/zope/proxy/__init__.py
index b2366ab..e195dc1 100644
--- a/src/zope/proxy/__init__.py
+++ b/src/zope/proxy/__init__.py
@@ -480,7 +480,7 @@ def py_removeAllProxies(obj):
 
 
 _c_available = False
-if 'PURE_PYTHON' not in os.environ:
+if not int(os.environ.get('PURE_PYTHON', '0')):
     try:  # pragma: no cover
         from zope.proxy._zope_proxy_proxy import ProxyBase as _c_available
     except ImportError: