diff --git a/Python/initconfig.c b/Python/initconfig.c
index c313d91..7662d61 100644
--- a/Python/initconfig.c
+++ b/Python/initconfig.c
@@ -68,6 +68,7 @@
 -X opt : set implementation-specific option. The following options are available:\n\
 \n\
          -X faulthandler: enable faulthandler\n\
+         -X oldparser: enable the traditional LL(1) parser; also PYTHONOLDPARSER\n\
          -X showrefcount: output the total reference count and number of used\n\
              memory blocks when the program finishes or after each statement in the\n\
              interactive interpreter. This only works on debug builds\n\
@@ -634,6 +635,7 @@
 #ifdef MS_WINDOWS
     config->legacy_windows_stdio = -1;
 #endif
+    config->use_peg = 1;
 }
 
 
@@ -791,6 +793,7 @@
     COPY_ATTR(isolated);
     COPY_ATTR(use_environment);
     COPY_ATTR(dev_mode);
+    COPY_ATTR(use_peg);
     COPY_ATTR(install_signal_handlers);
     COPY_ATTR(use_hash_seed);
     COPY_ATTR(hash_seed);
@@ -894,6 +897,7 @@
     SET_ITEM_INT(isolated);
     SET_ITEM_INT(use_environment);
     SET_ITEM_INT(dev_mode);
+    SET_ITEM_INT(use_peg);
     SET_ITEM_INT(install_signal_handlers);
     SET_ITEM_INT(use_hash_seed);
     SET_ITEM_UINT(hash_seed);
@@ -1428,6 +1432,11 @@
         config->import_time = 1;
     }
 
+    if (config_get_env(config, "PYTHONOLDPARSER")
+       || config_get_xoption(config, L"oldparser")) {
+        config->use_peg = 0;
+    }
+
     PyStatus status;
     if (config->tracemalloc < 0) {
         status = config_init_tracemalloc(config);
@@ -2507,6 +2516,7 @@
     assert(config->isolated >= 0);
     assert(config->use_environment >= 0);
     assert(config->dev_mode >= 0);
+    assert(config->use_peg >= 0);
     assert(config->install_signal_handlers >= 0);
     assert(config->use_hash_seed >= 0);
     assert(config->faulthandler >= 0);