diff --git a/tcmalloc/static_vars.cc b/tcmalloc/static_vars.cc
index 38fe08595..12e76c002 100644
--- a/tcmalloc/static_vars.cc
+++ b/tcmalloc/static_vars.cc
@@ -17,6 +17,7 @@
 #include <stddef.h>
 
 #include <atomic>
+#include <cstring>
 
 #include "absl/base/attributes.h"
 #include "absl/base/const_init.h"
@@ -172,16 +173,32 @@ int ABSL_ATTRIBUTE_WEAK default_want_legacy_size_classes();
 SizeClassConfiguration Static::size_class_configuration() {
   if (IsExperimentActive(Experiment::TEST_ONLY_TCMALLOC_POW2_SIZECLASS)) {
     return SizeClassConfiguration::kPow2Only;
-  } else if (default_want_legacy_size_classes != nullptr &&
-             default_want_legacy_size_classes() > 0) {
-    // TODO(b/242710633): remove this opt out.
+  }
+
+  // TODO(b/242710633): remove this opt out.
+  if (default_want_legacy_size_classes != nullptr &&
+      default_want_legacy_size_classes() > 0) {
     return SizeClassConfiguration::kLegacy;
-  } else if (IsExperimentActive(
-                 Experiment::TEST_ONLY_TCMALLOC_REUSE_SIZE_CLASSES)) {
+  }
+
+  if (IsExperimentActive(Experiment::TEST_ONLY_TCMALLOC_REUSE_SIZE_CLASSES)) {
     return SizeClassConfiguration::kReuse;
-  } else {
+  }
+
+  const char* e = thread_safe_getenv("TCMALLOC_LEGACY_SIZE_CLASSES");
+  if (e == nullptr) {
+    // TODO(b/358126781): Change this to use reuse size classes.
+    return SizeClassConfiguration::kPow2Below64;
+  } else if (!strcmp(e, "pow2below64")) {
     return SizeClassConfiguration::kPow2Below64;
+  } else if (!strcmp(e, "0")) {
+    // TODO(b/358126781): Change this to use reuse size classes.
+    return SizeClassConfiguration::kPow2Below64;
+  } else {
+    TC_BUG("bad TCMALLOC_LEGACY_SIZE_CLASSES env var '%s'", e);
   }
+  // TODO(b/358126781): Change this to use reuse size classes.
+  return SizeClassConfiguration::kPow2Below64;
 }
 
 ABSL_ATTRIBUTE_COLD ABSL_ATTRIBUTE_NOINLINE void Static::SlowInitIfNecessary() {
diff --git a/tcmalloc/testing/want_pow2below64_size_classes_helper.cc b/tcmalloc/testing/want_pow2below64_size_classes_helper.cc
new file mode 100644
index 000000000..1142b7ef5
--- /dev/null
+++ b/tcmalloc/testing/want_pow2below64_size_classes_helper.cc
@@ -0,0 +1,39 @@
+// Copyright 2019 The TCMalloc Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <cstdio>
+
+#include "absl/types/span.h"
+#include "tcmalloc/common.h"
+#include "tcmalloc/internal/logging.h"
+#include "tcmalloc/sizemap.h"
+#include "tcmalloc/static_vars.h"
+
+int main(int argc, char** argv) {
+  const size_t kExpectedClasses[] = {0, 8, 16, 32, 64, 72, 80, 88};
+
+  absl::Span<const size_t> classes = absl::MakeSpan(kExpectedClasses);
+
+  TC_CHECK_LE(classes.size(), tcmalloc::tcmalloc_internal::kNumClasses);
+  for (int c = 0; c < classes.size(); ++c) {
+    if (tcmalloc::tcmalloc_internal::Static::sizemap().class_to_size(c) !=
+        classes[c]) {
+      printf("Other");
+      return 0;
+    }
+  }
+  printf("Pow2Below64");
+
+  return 0;
+}