diff --git a/crossenv/scripts/site.py.tmpl b/crossenv/scripts/site.py.tmpl
index 14ee185..7b20cd0 100644
--- a/crossenv/scripts/site.py.tmpl
+++ b/crossenv/scripts/site.py.tmpl
@@ -9,7 +9,21 @@
 import sys
 import posix
 
-_sentinel = posix.environ.get(b"PYTHON_CROSSENV")
+# Handle subinterpreters -- everything should already be ready to go, but
+# the PYTHON_CROSSENV environment variable won't be set because we're running
+# in the same process
+_sentinel = None
+try:
+    import concurrent.interpreters
+except ImportError:
+    pass
+else:
+    if concurrent.interpreters.get_current() != concurrent.interpreters.get_main():
+        _sentinel = b"{{context.sentinel}}"
+
+
+if _sentinel is None:
+    _sentinel = posix.environ.get(b"PYTHON_CROSSENV")
 if _sentinel != b"{{context.sentinel}}":
     print("*******************************************************", file=sys.stderr)
     print("* Crossenv has leaked into another Python interpreter!", file=sys.stderr)
diff --git a/tests/test_environment.py b/tests/test_environment.py
index 5cb11b5..d9a4e80 100644
--- a/tests/test_environment.py
+++ b/tests/test_environment.py
@@ -1,5 +1,6 @@
 import os
 import re
+import subprocess
 from textwrap import dedent
 
 import pytest
@@ -218,6 +219,30 @@ def test_environment_leak(crossenv):
             ),
         ],
         universal_newlines=True,
+        stderr=subprocess.STDOUT,
+    )
+    assert "Crossenv has leaked" not in out
+
+
+def test_environment_subinterpreter_leak(crossenv):
+    pytest.importorskip("concurrent.interpreters")
+    out = crossenv.check_output(
+        [
+            "python",
+            "-c",
+            dedent(
+                """\
+            import concurrent.interpreters
+
+            def fn():
+                print("ok")
+
+            concurrent.interpreters.create().call(fn)
+            """
+            ),
+        ],
+        universal_newlines=True,
+        stderr=subprocess.STDOUT,
     )
     assert "Crossenv has leaked" not in out