diff --git a/src/main/java/com/google/devtools/build/lib/rules/python/PythonOptions.java b/src/main/java/com/google/devtools/build/lib/rules/python/PythonOptions.java
index f69cd517b94600..e14b36f76af2bb 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/python/PythonOptions.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/python/PythonOptions.java
@@ -231,7 +231,7 @@ public String getTypeDescription() {
 
   @Option(
       name = "incompatible_use_python_toolchains",
-      defaultValue = "true",
+      defaultValue = "false",
       documentationCategory = OptionDocumentationCategory.GENERIC_INPUTS,
       effectTags = {OptionEffectTag.LOADING_AND_ANALYSIS},
       metadataTags = {
diff --git a/src/test/java/com/google/devtools/build/lib/bazel/rules/python/BazelPyBinaryConfiguredTargetTest.java b/src/test/java/com/google/devtools/build/lib/bazel/rules/python/BazelPyBinaryConfiguredTargetTest.java
index 7fbe0e1982ac0e..a878000b0a28bb 100644
--- a/src/test/java/com/google/devtools/build/lib/bazel/rules/python/BazelPyBinaryConfiguredTargetTest.java
+++ b/src/test/java/com/google/devtools/build/lib/bazel/rules/python/BazelPyBinaryConfiguredTargetTest.java
@@ -79,8 +79,6 @@ private String getInterpreterPathFromStub(ConfiguredTarget pyExecutableTarget) {
         "Failed to find the '%python_binary%' key in the stub script's template expansion action");
   }
 
-  // TODO(#8169): Delete tests of the legacy --python_top / --python_path behavior.
-
   @Test
   public void runtimeSetByPythonTop() throws Exception {
     scratch.file(
diff --git a/src/test/java/com/google/devtools/build/lib/bazel/rules/python/BazelPythonConfigurationTest.java b/src/test/java/com/google/devtools/build/lib/bazel/rules/python/BazelPythonConfigurationTest.java
index 6849c39c902f3c..5b5eb2dfa72651 100644
--- a/src/test/java/com/google/devtools/build/lib/bazel/rules/python/BazelPythonConfigurationTest.java
+++ b/src/test/java/com/google/devtools/build/lib/bazel/rules/python/BazelPythonConfigurationTest.java
@@ -33,9 +33,8 @@ public void pythonTop() throws Exception {
     scratch.file(
         "a/BUILD",
         "py_runtime(name='b', files=[], interpreter='c')");
-    BazelPythonConfiguration config =
-        create("--incompatible_use_python_toolchains=false", "--python_top=//a:b")
-            .getFragment(BazelPythonConfiguration.class);
+    BazelPythonConfiguration config = create("--python_top=//a:b")
+        .getFragment(BazelPythonConfiguration.class);
     assertThat(config.getPythonTop()).isNotNull();
   }
 
@@ -44,7 +43,7 @@ public void pythonTop_malformedLabel() {
     OptionsParsingException expected =
         assertThrows(
             OptionsParsingException.class,
-            () -> create("--incompatible_use_python_toolchains=false", "--python_top=//a:!b:"));
+            () -> create("--python_top=//a:!b:"));
     assertThat(expected).hasMessageThat().contains("While parsing option --python_top");
   }
 
diff --git a/src/test/java/com/google/devtools/build/lib/packages/util/BazelMockPythonSupport.java b/src/test/java/com/google/devtools/build/lib/packages/util/BazelMockPythonSupport.java
index 3a39c71fdf4a80..ae665ffc6871f1 100644
--- a/src/test/java/com/google/devtools/build/lib/packages/util/BazelMockPythonSupport.java
+++ b/src/test/java/com/google/devtools/build/lib/packages/util/BazelMockPythonSupport.java
@@ -54,24 +54,10 @@ public void setup(MockToolsConfig config) throws IOException {
         "toolchain_type(name = 'toolchain_type')",
         "constraint_setting(name = 'py2_interpreter_path')",
         "constraint_setting(name = 'py3_interpreter_path')",
-        "py_runtime(",
-        "    name = 'py2_interpreter',",
-        "    interpreter_path = '/usr/bin/mockpython2',",
-        "    python_version = 'PY2',",
-        ")",
-        "py_runtime(",
-        "    name = 'py3_interpreter',",
-        "    interpreter_path = '/usr/bin/mockpython3',",
-        "    python_version = 'PY3',",
-        ")",
-        "py_runtime_pair(",
-        "    name = 'default_py_runtime_pair',",
-        "    py2_runtime = ':py2_interpreter',",
-        "    py3_runtime = ':py3_interpreter',",
-        ")",
+        "py_runtime_pair(name = 'dummy_py_runtime_pair')",
         "toolchain(",
-        "    name = 'default_python_toolchain',",
-        "    toolchain = ':default_py_runtime_pair',",
+        "    name = 'dummy_toolchain',",
+        "    toolchain = ':dummy_py_runtime_pair',",
         "    toolchain_type = ':toolchain_type',",
         ")",
         "exports_files(['precompile.py'])",
diff --git a/src/test/java/com/google/devtools/build/lib/rules/python/BUILD b/src/test/java/com/google/devtools/build/lib/rules/python/BUILD
index caabb0862665b1..2e81f708b04f05 100644
--- a/src/test/java/com/google/devtools/build/lib/rules/python/BUILD
+++ b/src/test/java/com/google/devtools/build/lib/rules/python/BUILD
@@ -254,7 +254,6 @@ java_test(
         "//src/main/java/com/google/devtools/build/lib:python-rules",
         "//src/main/java/com/google/devtools/build/lib/actions",
         "//src/test/java/com/google/devtools/build/lib:analysis_testutil",
-        "//src/test/java/com/google/devtools/build/lib:testutil",
         "//third_party:junit4",
         "//third_party:truth",
     ],
diff --git a/src/test/java/com/google/devtools/build/lib/rules/python/PythonStarlarkApiTest.java b/src/test/java/com/google/devtools/build/lib/rules/python/PythonStarlarkApiTest.java
index 28c4ffe73dff38..a46772335c587e 100644
--- a/src/test/java/com/google/devtools/build/lib/rules/python/PythonStarlarkApiTest.java
+++ b/src/test/java/com/google/devtools/build/lib/rules/python/PythonStarlarkApiTest.java
@@ -20,7 +20,6 @@
 import com.google.devtools.build.lib.analysis.ConfiguredTarget;
 import com.google.devtools.build.lib.analysis.util.BuildViewTestCase;
 import com.google.devtools.build.lib.packages.StructImpl;
-import com.google.devtools.build.lib.testutil.TestConstants;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.junit.runners.JUnit4;
@@ -175,10 +174,6 @@ public void runtimeSandwich() throws Exception {
         ")");
     scratch.file(
         "pkg/BUILD",
-        "load('"
-            + TestConstants.TOOLS_REPOSITORY
-            + "//tools/python:toolchain.bzl', "
-            + "'py_runtime_pair')",
         "load(':rules.bzl', 'userruntime')",
         "py_runtime(",
         "    name = 'pyruntime',",
@@ -192,22 +187,13 @@ public void runtimeSandwich() throws Exception {
         "    interpreter = ':userintr',",
         "    files = ['userdata.txt'],",
         ")",
-        "py_runtime_pair(",
-        "    name = 'userruntime_pair',",
-        "    py2_runtime = 'userruntime',",
-        ")",
-        "toolchain(",
-        "    name = 'usertoolchain',",
-        "    toolchain = ':userruntime_pair',",
-        "    toolchain_type = '"
-            + TestConstants.TOOLS_REPOSITORY
-            + "//tools/python:toolchain_type',",
-        ")",
         "py_binary(",
         "    name = 'pybin',",
         "    srcs = ['pybin.py'],",
         ")");
-    useConfiguration("--extra_toolchains=//pkg:usertoolchain");
+    String pythonTopLabel =
+        analysisMock.pySupport().createPythonTopEntryPoint(mockToolsConfig, "//pkg:userruntime");
+    useConfiguration("--python_top=" + pythonTopLabel);
     ConfiguredTarget target = getConfiguredTarget("//pkg:pybin");
     assertThat(collectRunfiles(target))
         .containsAtLeast(getSourceArtifact("pkg/data.txt"), getSourceArtifact("pkg/userdata.txt"));
diff --git a/src/test/shell/bazel/android/android_instrumentation_test_integration_test.sh b/src/test/shell/bazel/android/android_instrumentation_test_integration_test.sh
index 2239ad46683a30..36f76b17fffa8d 100755
--- a/src/test/shell/bazel/android/android_instrumentation_test_integration_test.sh
+++ b/src/test/shell/bazel/android/android_instrumentation_test_integration_test.sh
@@ -29,11 +29,6 @@ fail_if_no_android_sdk
 source "${CURRENT_DIR}/../../integration_test_setup.sh" \
   || { echo "integration_test_setup.sh not found!" >&2; exit 1; }
 
-# TODO(#8169): Make this test compatible with Python toolchains. Blocked on the
-# fact that there's no PY3 environment on our Mac workers
-# (bazelbuild/continuous-integration#578).
-add_to_bazelrc "build --incompatible_use_python_toolchains=false"
-
 function setup_android_instrumentation_test_env() {
   mkdir -p java/com/bin/res/values
   mkdir -p javatests/com/bin
diff --git a/src/test/shell/bazel/python_version_test.sh b/src/test/shell/bazel/python_version_test.sh
index e1e1f27ce20601..e463f4d9531576 100755
--- a/src/test/shell/bazel/python_version_test.sh
+++ b/src/test/shell/bazel/python_version_test.sh
@@ -43,6 +43,9 @@ fi
 source "$(rlocation "io_bazel/src/test/shell/integration_test_setup.sh")" \
   || { echo "integration_test_setup.sh not found!" >&2; exit 1; }
 
+# TODO(bazelbuild/continuous-integration#578): Enable this test for Mac and
+# Windows.
+
 # `uname` returns the current platform, e.g "MSYS_NT-10.0" or "Linux".
 # `tr` converts all upper case letters to lower case.
 # `case` matches the result if the `uname | tr` expression to string prefixes
@@ -52,15 +55,15 @@ case "$(uname -s | tr [:upper:] [:lower:])" in
 msys*)
   # As of 2018-08-14, Bazel on Windows only supports MSYS Bash.
   declare -r is_windows=true
-  # As of 2019-04-26, this test is disabled on Windows (via "no_windows" tag),
-  # so this code shouldn't even have run.
-  # TODO(bazelbuild/continuous-integration#578): Enable this test for Windows.
+  # As of 2018-12-17, this test is disabled on windows (via "no_windows" tag),
+  # so this code shouldn't even have run. See the TODO at
+  # use_system_python_2_3_runtimes.
   fail "This test does not run on Windows."
   ;;
 darwin*)
-  # As of 2019-04-26, this test is disabled on mac, but there's no "no_mac" tag
-  # so we just have to trivially succeed.
-  # TODO(bazelbuild/continuous-integration#578): Enable this test for Mac.
+  # As of 2018-12-17, this test is disabled on mac, but there's no "no_mac" tag
+  # so we just have to trivially succeed. See the TODO at
+  # use_system_python_2_3_runtimes.
   echo "This test does not run on Mac; exiting early." >&2
   exit 0
   ;;
@@ -76,6 +79,44 @@ if "$is_windows"; then
   export MSYS2_ARG_CONV_EXCL="*"
 fi
 
+# Use a py_runtime that invokes either the system's Python 2 or Python 3
+# interpreter based on the Python mode. On Unix this is a workaround for #4815.
+#
+# TODO(brandjon): Replace this with the autodetecting Python toolchain.
+function use_system_python_2_3_runtimes() {
+  PYTHON2_BIN=$(which python2 || echo "")
+  PYTHON3_BIN=$(which python3 || echo "")
+  # Debug output.
+  echo "Python 2 interpreter: ${PYTHON2_BIN:-"Not found"}"
+  echo "Python 3 interpreter: ${PYTHON3_BIN:-"Not found"}"
+  # Fail if either isn't present.
+  if [[ -z "${PYTHON2_BIN:-}" || -z "${PYTHON3_BIN:-}" ]]; then
+    fail "Can't use system interpreter: Could not find one or both of \
+'python2', 'python3'"
+  fi
+
+  # Point Python builds at a py_runtime target defined in a //tools package of
+  # the main repo. This is not related to @bazel_tools//tools/python.
+  add_to_bazelrc "build --python_top=//tools/python:default_runtime"
+
+  mkdir -p tools/python
+
+  cat > tools/python/BUILD << EOF
+package(default_visibility=["//visibility:public"])
+
+py_runtime(
+    name = "default_runtime",
+    files = [],
+    interpreter_path = select({
+        "@bazel_tools//tools/python:PY2": "${PYTHON2_BIN}",
+        "@bazel_tools//tools/python:PY3": "${PYTHON3_BIN}",
+    }),
+)
+EOF
+}
+
+use_system_python_2_3_runtimes
+
 #### TESTS #############################################################
 
 # Sanity test that our environment setup above works.
@@ -161,8 +202,6 @@ function test_build_python_zip_works_with_py_runtime() {
   mkdir -p test
 
   cat > test/BUILD << EOF
-load("@bazel_tools//tools/python:toolchain.bzl", "py_runtime_pair")
-
 py_binary(
     name = "pybin",
     srcs = ["pybin.py"],
@@ -173,17 +212,6 @@ py_runtime(
     interpreter = ":mockpy.sh",
     python_version = "PY3",
 )
-
-py_runtime_pair(
-    name = "mock_runtime_pair",
-    py3_runtime = ":mock_runtime",
-)
-
-toolchain(
-    name = "mock_toolchain",
-    toolchain = ":mock_runtime_pair",
-    toolchain_type = "@bazel_tools//tools/python:toolchain_type",
-)
 EOF
   cat > test/pybin.py << EOF
 # This doesn't actually run because we use a mock Python runtime that never
@@ -196,8 +224,7 @@ echo "I am mockpy!"
 EOF
   chmod u+x test/mockpy.sh
 
-  bazel run //test:pybin \
-      --extra_toolchains=//test:mock_toolchain --build_python_zip \
+  bazel run //test:pybin --python_top=//test:mock_runtime --build_python_zip \
       &> $TEST_log || fail "bazel run failed"
   expect_log "I am mockpy!"
 }
diff --git a/src/test/shell/integration/runfiles_test.sh b/src/test/shell/integration/runfiles_test.sh
index 6e6f68bc8dc6d0..b089adfb4acf3a 100755
--- a/src/test/shell/integration/runfiles_test.sh
+++ b/src/test/shell/integration/runfiles_test.sh
@@ -51,18 +51,14 @@ msys*|mingw*|cygwin*)
   ;;
 esac
 
-# We disable Python toolchains in EXTRA_BUILD_FLAGS because it throws off the
-# counts and manifest checks in test_foo_runfiles.
-# TODO(#8169): Update this test and remove the toolchain opt-out.
 if "$is_windows"; then
   export MSYS_NO_PATHCONV=1
   export MSYS2_ARG_CONV_EXCL="*"
   export EXT=".exe"
-  export EXTRA_BUILD_FLAGS="--incompatible_use_python_toolchains=false \
---enable_runfiles --build_python_zip=0"
+  export EXTRA_BUILD_FLAGS="--enable_runfiles --build_python_zip=0"
 else
   export EXT=""
-  export EXTRA_BUILD_FLAGS="--incompatible_use_python_toolchains=false"
+  export EXTRA_BUILD_FLAGS=""
 fi
 
 #### SETUP #############################################################
diff --git a/src/test/shell/testenv.sh b/src/test/shell/testenv.sh
index 999e7321b2eee8..a3717f263514df 100755
--- a/src/test/shell/testenv.sh
+++ b/src/test/shell/testenv.sh
@@ -569,13 +569,11 @@ function use_fake_python_runtimes_for_testsuite() {
     PYTHON3_FILENAME="python3.sh"
   fi
 
-  add_to_bazelrc "build --extra_toolchains=//tools/python:fake_python_toolchain"
+  add_to_bazelrc "build --python_top=//tools/python:default_runtime"
 
   mkdir -p tools/python
 
   cat > tools/python/BUILD << EOF
-load("@bazel_tools//tools/python:toolchain.bzl", "py_runtime_pair")
-
 package(default_visibility=["//visibility:public"])
 
 sh_binary(
@@ -584,27 +582,15 @@ sh_binary(
 )
 
 py_runtime(
-    name = "fake_py2_interpreter",
-    interpreter = ":${PYTHON2_FILENAME}",
-    python_version = "PY2",
-)
-
-py_runtime(
-    name = "fake_py3_interpreter",
-    interpreter = ":${PYTHON3_FILENAME}",
-    python_version = "PY3",
-)
-
-py_runtime_pair(
-    name = "fake_py_runtime_pair",
-    py2_runtime = ":fake_py2_interpreter",
-    py3_runtime = ":fake_py3_interpreter",
-)
-
-toolchain(
-    name = "fake_python_toolchain",
-    toolchain = ":fake_py_runtime_pair",
-    toolchain_type = "@bazel_tools//tools/python:toolchain_type",
+    name = "default_runtime",
+    files = select({
+        "@bazel_tools//tools/python:PY3": [":${PYTHON3_FILENAME}"],
+        "//conditions:default": [":${PYTHON2_FILENAME}"],
+    }),
+    interpreter = select({
+        "@bazel_tools//tools/python:PY3": ":${PYTHON3_FILENAME}",
+        "//conditions:default": ":${PYTHON2_FILENAME}",
+    }),
 )
 EOF