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)