diff --git a/psycopg/connection_int.c b/psycopg/connection_int.c
index de7083f7..0544957f 100644
--- a/psycopg/connection_int.c
+++ b/psycopg/connection_int.c
@@ -1042,6 +1042,13 @@ conn_switch_isolation_level(connectionObject *self, int level)
 {
     int curr_level;
 
+    /* use only supported levels on older PG versions */
+    if (self->server_version < 80000) {
+        if (level == 1 || level == 3) {
+            ++level;
+        }
+    }
+
     if (-1 == (curr_level = conn_get_isolation_level(self))) {
         return -1;
     }
diff --git a/psycopg/connection_type.c b/psycopg/connection_type.c
index 455abdae..3a5b7feb 100644
--- a/psycopg/connection_type.c
+++ b/psycopg/connection_type.c
@@ -384,10 +384,9 @@ psyco_conn_tpc_recover(connectionObject *self, PyObject *args)
 extern const IsolationLevel conn_isolevels[];
 
 static const char *
-_psyco_conn_parse_isolevel(PyObject *pyval)
+_psyco_conn_parse_isolevel(connectionObject *self, PyObject *pyval)
 {
-    const char *rv = NULL;
-    const IsolationLevel *value = NULL;
+    const IsolationLevel *isolevel = NULL;
 
     Py_INCREF(pyval);   /* for ensure_bytes */
 
@@ -400,22 +399,22 @@ _psyco_conn_parse_isolevel(PyObject *pyval)
                 "isolation_level must be between 1 and 4");
             goto exit;
         }
-        rv = conn_isolevels[level].name;
+
+        isolevel = conn_isolevels + level;
     }
 
     /* parse from the string -- this includes "default" */
     else {
-        value = conn_isolevels;
-        while ((++value)->name) {
+        isolevel = conn_isolevels;
+        while ((++isolevel)->name) {
             if (!(pyval = psycopg_ensure_bytes(pyval))) {
                 goto exit;
             }
-            if (0 == strcasecmp(value->name, Bytes_AS_STRING(pyval))) {
-                rv = value->name;
+            if (0 == strcasecmp(isolevel->name, Bytes_AS_STRING(pyval))) {
                 break;
             }
         }
-        if (!rv) {
+        if (!isolevel->name) {
             char msg[256];
             snprintf(msg, sizeof(msg),
                 "bad value for isolation_level: '%s'", Bytes_AS_STRING(pyval));
@@ -423,9 +422,17 @@ _psyco_conn_parse_isolevel(PyObject *pyval)
         }
     }
 
+    /* use only supported levels on older PG versions */
+    if (isolevel && self->server_version < 80000) {
+        if (isolevel->value == 1 || isolevel->value == 3) {
+            ++isolevel;
+        }
+    }
+
 exit:
     Py_XDECREF(pyval);
-    return rv;
+
+    return isolevel ? isolevel->name : NULL;
 }
 
 /* convert True/False/"default" into a C string */
@@ -477,7 +484,7 @@ psyco_conn_set_transaction(connectionObject *self, PyObject *args, PyObject *kwa
 
     if (Py_None != isolation_level) {
         const char *value = NULL;
-        if (!(value = _psyco_conn_parse_isolevel(isolation_level))) {
+        if (!(value = _psyco_conn_parse_isolevel(self, isolation_level))) {
             return NULL;
         }
         if (0 != conn_set(self, "default_transaction_isolation", value)) {
diff --git a/tests/test_connection.py b/tests/test_connection.py
index 90014574..e62e51e3 100755
--- a/tests/test_connection.py
+++ b/tests/test_connection.py
@@ -204,14 +204,23 @@ class IsolationLevelsTestCase(unittest.TestCase):
         conn = self.connect()
         curs = conn.cursor()
 
-        for name, level in (
+        levels = (
             (None, psycopg2.extensions.ISOLATION_LEVEL_AUTOCOMMIT),
             ('read uncommitted', psycopg2.extensions.ISOLATION_LEVEL_READ_UNCOMMITTED),
             ('read committed', psycopg2.extensions.ISOLATION_LEVEL_READ_COMMITTED),
             ('repeatable read', psycopg2.extensions.ISOLATION_LEVEL_REPEATABLE_READ),
             ('serializable', psycopg2.extensions.ISOLATION_LEVEL_SERIALIZABLE),
-        ):
+        )
+        for name, level in levels:
             conn.set_isolation_level(level)
+
+            # the only values available on prehistoric PG versions
+            if conn.server_version < 80000:
+                if level in (
+                        psycopg2.extensions.ISOLATION_LEVEL_READ_UNCOMMITTED,
+                        psycopg2.extensions.ISOLATION_LEVEL_REPEATABLE_READ):
+                    name, level = levels[levels.index((name, level)) + 1]
+
             self.assertEqual(conn.isolation_level, level)
 
             curs.execute('show transaction_isolation;')