diff --git a/src/backend/commands/variable.c b/src/backend/commands/variable.c
index 9a754dae3fd..c2d7a5bebf6 100644
--- a/src/backend/commands/variable.c
+++ b/src/backend/commands/variable.c
@@ -522,32 +522,9 @@ check_transaction_read_only(bool *newval, void **extra, GucSource source)
  * As in check_transaction_read_only, allow it if not inside a transaction.
  */
 bool
-check_XactIsoLevel(char **newval, void **extra, GucSource source)
+check_XactIsoLevel(int *newval, void **extra, GucSource source)
 {
-	int			newXactIsoLevel;
-
-	if (strcmp(*newval, "serializable") == 0)
-	{
-		newXactIsoLevel = XACT_SERIALIZABLE;
-	}
-	else if (strcmp(*newval, "repeatable read") == 0)
-	{
-		newXactIsoLevel = XACT_REPEATABLE_READ;
-	}
-	else if (strcmp(*newval, "read committed") == 0)
-	{
-		newXactIsoLevel = XACT_READ_COMMITTED;
-	}
-	else if (strcmp(*newval, "read uncommitted") == 0)
-	{
-		newXactIsoLevel = XACT_READ_UNCOMMITTED;
-	}
-	else if (strcmp(*newval, "default") == 0)
-	{
-		newXactIsoLevel = DefaultXactIsoLevel;
-	}
-	else
-		return false;
+	int			newXactIsoLevel = *newval;
 
 	if (newXactIsoLevel != XactIsoLevel && IsTransactionState())
 	{
@@ -574,39 +551,9 @@ check_XactIsoLevel(char **newval, void **extra, GucSource source)
 		}
 	}
 
-	*extra = malloc(sizeof(int));
-	if (!*extra)
-		return false;
-	*((int *) *extra) = newXactIsoLevel;
-
 	return true;
 }
 
-void
-assign_XactIsoLevel(const char *newval, void *extra)
-{
-	XactIsoLevel = *((int *) extra);
-}
-
-const char *
-show_XactIsoLevel(void)
-{
-	/* We need this because we don't want to show "default". */
-	switch (XactIsoLevel)
-	{
-		case XACT_READ_UNCOMMITTED:
-			return "read uncommitted";
-		case XACT_READ_COMMITTED:
-			return "read committed";
-		case XACT_REPEATABLE_READ:
-			return "repeatable read";
-		case XACT_SERIALIZABLE:
-			return "serializable";
-		default:
-			return "bogus";
-	}
-}
-
 /*
  * SET TRANSACTION [NOT] DEFERRABLE
  */
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 0bec3914f80..2317e8be6be 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -515,7 +515,6 @@ static int	server_version_num;
 static char *timezone_string;
 static char *log_timezone_string;
 static char *timezone_abbreviations_string;
-static char *XactIsoLevel_string;
 static char *data_directory;
 static char *session_authorization_string;
 static int	max_function_args;
@@ -3618,17 +3617,6 @@ static struct config_string ConfigureNamesString[] =
 		check_timezone_abbreviations, assign_timezone_abbreviations, NULL
 	},
 
-	{
-		{"transaction_isolation", PGC_USERSET, CLIENT_CONN_STATEMENT,
-			gettext_noop("Sets the current transaction's isolation level."),
-			NULL,
-			GUC_NO_RESET_ALL | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
-		},
-		&XactIsoLevel_string,
-		"default",
-		check_XactIsoLevel, assign_XactIsoLevel, show_XactIsoLevel
-	},
-
 	{
 		{"unix_socket_group", PGC_POSTMASTER, CONN_AUTH_SETTINGS,
 			gettext_noop("Sets the owning group of the Unix-domain socket."),
@@ -3968,6 +3956,17 @@ static struct config_enum ConfigureNamesEnum[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"transaction_isolation", PGC_USERSET, CLIENT_CONN_STATEMENT,
+			gettext_noop("Sets the current transaction's isolation level."),
+			NULL,
+			GUC_NO_RESET_ALL | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+		},
+		&XactIsoLevel,
+		XACT_READ_COMMITTED, isolation_level_options,
+		check_XactIsoLevel, NULL, NULL
+	},
+
 	{
 		{"IntervalStyle", PGC_USERSET, CLIENT_CONN_LOCALE,
 			gettext_noop("Sets the display format for interval values."),
@@ -4776,7 +4775,7 @@ InitializeGUCOptions(void)
 	 * Prevent any attempt to override the transaction modes from
 	 * non-interactive sources.
 	 */
-	SetConfigOption("transaction_isolation", "default",
+	SetConfigOption("transaction_isolation", "read committed",
 					PGC_POSTMASTER, PGC_S_OVERRIDE);
 	SetConfigOption("transaction_read_only", "no",
 					PGC_POSTMASTER, PGC_S_OVERRIDE);
diff --git a/src/include/commands/variable.h b/src/include/commands/variable.h
index 4ea3b0209b1..7373a3f99f8 100644
--- a/src/include/commands/variable.h
+++ b/src/include/commands/variable.h
@@ -22,9 +22,7 @@ extern bool check_log_timezone(char **newval, void **extra, GucSource source);
 extern void assign_log_timezone(const char *newval, void *extra);
 extern const char *show_log_timezone(void);
 extern bool check_transaction_read_only(bool *newval, void **extra, GucSource source);
-extern bool check_XactIsoLevel(char **newval, void **extra, GucSource source);
-extern void assign_XactIsoLevel(const char *newval, void *extra);
-extern const char *show_XactIsoLevel(void);
+extern bool check_XactIsoLevel(int *newval, void **extra, GucSource source);
 extern bool check_transaction_deferrable(bool *newval, void **extra, GucSource source);
 extern bool check_random_seed(double *newval, void **extra, GucSource source);
 extern void assign_random_seed(double newval, void *extra);