diff --git a/Lib/importlib/_bootstrap.py b/Lib/importlib/_bootstrap.py
--- a/Lib/importlib/_bootstrap.py
+++ b/Lib/importlib/_bootstrap.py
@@ -33,7 +33,10 @@ def _make_relax_case():
     if sys.platform.startswith(_CASE_INSENSITIVE_PLATFORMS):
         def _relax_case():
             """True if filenames must be checked case-insensitively."""
-            return b'PYTHONCASEOK' in _os.environ
+            if sys.flags.ignore_environment:
+                return False
+            else:
+                return b'PYTHONCASEOK' in _os.environ
     else:
         def _relax_case():
             """True if filenames must be checked case-insensitively."""
diff --git a/Lib/test/test_importlib/extension/test_case_sensitivity.py b/Lib/test/test_importlib/extension/test_case_sensitivity.py
--- a/Lib/test/test_importlib/extension/test_case_sensitivity.py
+++ b/Lib/test/test_importlib/extension/test_case_sensitivity.py
@@ -5,7 +5,8 @@ import unittest
 from importlib import _bootstrap
 from .. import util
 from . import util as ext_util
-
+import os
+import subprocess
 
 @util.case_insensitive_tests
 class ExtensionModuleCaseSensitivityTest(unittest.TestCase):
@@ -29,14 +30,34 @@ class ExtensionModuleCaseSensitivityTest
             self.assertIsNone(loader)
 
     def test_case_insensitivity(self):
-        with support.EnvironmentVarGuard() as env:
-            env.set('PYTHONCASEOK', '1')
-            if b'PYTHONCASEOK' not in _bootstrap._os.environ:
-                self.skipTest('os.environ changes not reflected in '
-                              '_os.environ')
-            loader = self.find_module()
-            self.assertTrue(hasattr(loader, 'load_module'))
+        find_snippet = """if True:
+            from importlib import _bootstrap
+            import sys
+            finder = _bootstrap.FileFinder('{path}',
+                                           (_bootstrap.ExtensionFileLoader,
+                                            _bootstrap.EXTENSION_SUFFIXES))
+            loader = finder.find_module('{bad_name}')
+            print(str(hasattr(loader, 'load_module')))
+            """.format(bad_name=ext_util.NAME.upper(), path=ext_util.PATH)
 
+        newenv = os.environ.copy()
+        newenv["PYTHONCASEOK"] = "1"
+
+        def check_output(expected, extra_arg=None):
+            args = [sys.executable]
+            if extra_arg:
+                args.append(extra_arg)
+            args.extend(["-c", find_snippet])
+            p = subprocess.Popen(args, stdout=subprocess.PIPE, env=newenv)
+            actual = p.communicate()[0].decode().strip()
+            self.assertEqual(expected, actual)
+            self.assertEqual(p.wait(), 0)
+
+        # Test with PYTHONCASEOK=1.
+        check_output("True")
+
+        # Test with PYTHONCASEOK=1 ignored because of -E.
+        check_output("False", "-E")
 
 
 
diff --git a/Lib/test/test_importlib/source/test_case_sensitivity.py b/Lib/test/test_importlib/source/test_case_sensitivity.py
--- a/Lib/test/test_importlib/source/test_case_sensitivity.py
+++ b/Lib/test/test_importlib/source/test_case_sensitivity.py
@@ -8,6 +8,7 @@ import os
 import sys
 from test import support as test_support
 import unittest
+import subprocess
 
 
 @util.case_insensitive_tests
@@ -50,16 +51,62 @@ class CaseSensitivityTest(unittest.TestC
             self.assertIsNone(insensitive)
 
     def test_insensitive(self):
-        with test_support.EnvironmentVarGuard() as env:
-            env.set('PYTHONCASEOK', '1')
-            if b'PYTHONCASEOK' not in _bootstrap._os.environ:
-                self.skipTest('os.environ changes not reflected in '
-                              '_os.environ')
-            sensitive, insensitive = self.sensitivity_test()
-            self.assertTrue(hasattr(sensitive, 'load_module'))
-            self.assertIn(self.name, sensitive.get_filename(self.name))
-            self.assertTrue(hasattr(insensitive, 'load_module'))
-            self.assertIn(self.name, insensitive.get_filename(self.name))
+        sensitive_pkg = 'sensitive.{0}'.format(self.name)
+        insensitive_pkg = 'insensitive.{0}'.format(self.name.lower())
+        context = source_util.create_modules(insensitive_pkg, sensitive_pkg)
+        with context as mapping:
+            sensitive_path = os.path.join(mapping['.root'], 'sensitive')
+            insensitive_path = os.path.join(mapping['.root'], 'insensitive')
+            find_snippet = """if True:
+                import sys
+                from importlib import machinery
+
+                def find(path):
+                    f = machinery.FileFinder(path,
+                                             (machinery.SourceFileLoader,
+                                              machinery.SOURCE_SUFFIXES),
+                                             (machinery.SourcelessFileLoader,
+                                              machinery.BYTECODE_SUFFIXES))
+                    return f.find_module('{name}')
+
+                sensitive = find('{sensitive_path}')
+                insensitive = find('{insensitive_path}')
+                print(str(hasattr(sensitive, 'load_module')))
+                if hasattr(sensitive, 'load_module'):
+                    print(sensitive.get_filename('{name}'))
+                else:
+                    print('None')
+                print(str(hasattr(insensitive, 'load_module')))
+                if hasattr(insensitive, 'load_module'):
+                    print(insensitive.get_filename('{name}'))
+                else:
+                    print('None')
+                """.format(sensitive_path=sensitive_path,
+                           insensitive_path=insensitive_path,
+                           name=self.name)
+
+            newenv = os.environ.copy()
+            newenv["PYTHONCASEOK"] = "1"
+
+            def check_output(expected, extra_arg=None):
+                args = [sys.executable]
+                if extra_arg:
+                    args.append(extra_arg)
+                args.extend(["-c", find_snippet])
+                p = subprocess.Popen(args, stdout=subprocess.PIPE,
+                                         env=newenv)
+                actual = p.communicate()[0].decode().split()
+                self.assertEqual(expected[0], actual[0])
+                self.assertIn(expected[1], actual[1])
+                self.assertEqual(expected[2], actual[2])
+                self.assertIn(expected[3], actual[3])
+                self.assertEqual(p.wait(), 0)
+
+            # Test with PYTHONCASEOK=1.
+            check_output(["True", self.name, "True", self.name])
+
+            # Test with PYTHONCASEOK=1 ignored because of -E.
+            check_output(["True", self.name, "False", "None"], "-E")
 
 
 def test_main():