diff --git a/Lib/test/test_site.py b/Lib/test/test_site.py

index d0cd84f..9d2c28c 100644

--- a/Lib/test/test_site.py

+++ b/Lib/test/test_site.py

@@ -10,6 +10,7 @@ from test import support

 from test.support import (captured_stderr, TESTFN, EnvironmentVarGuard,

                           change_cwd)

 import builtins

+import glob

 import os

 import sys

 import re

@@ -500,6 +501,23 @@ class ImportSideEffectTests(unittest.TestCase):

 class StartupImportTests(unittest.TestCase):

     def test_startup_imports(self):

+        # Get sys.path in isolated mode (python3 -I)

+        popen = subprocess.Popen([sys.executable, '-I', '-c',

+                                  'import sys; print(repr(sys.path))'],

+                                 stdout=subprocess.PIPE,

+                                 encoding='utf-8')

+        stdout = popen.communicate()[0]

+        self.assertEqual(popen.returncode, 0, repr(stdout))

+        isolated_paths = eval(stdout)

+

+        # bpo-27807: Even with -I, the site module executes all .pth files

+        # found in sys.path (see site.addpackage()). Skip the test if at least

+        # one .pth file is found.

+        for path in isolated_paths:

+            pth_files = glob.glob(os.path.join(path, "*.pth"))

+            if pth_files:

+                self.skipTest(f"found {len(pth_files)} .pth files in: {path}")

+

         # This tests checks which modules are loaded by Python when it

         # initially starts upon startup.

         popen = subprocess.Popen([sys.executable, '-I', '-v', '-c',

@@ -508,6 +526,7 @@ class StartupImportTests(unittest.TestCase):

                                  stderr=subprocess.PIPE,

                                  encoding='utf-8')

         stdout, stderr = popen.communicate()

+        self.assertEqual(popen.returncode, 0, (stdout, stderr))

         modules = eval(stdout)

         self.assertIn('site', modules)