diff --git a/NEWS b/NEWS
index a1085252..1f1d7c85 100644
--- a/NEWS
+++ b/NEWS
@@ -24,6 +24,7 @@ What's new in psycopg 2.7.4
   (:ticket:`632`).
 - Fixed `~cursor.rowcount` after `~cursor.executemany()` with :sql:`RETURNING` statements
   (:ticket:`633`).
+- Fixed compatibility problem with pypy3 (:ticket:`#649`).
 - Wheel packages compiled against PostgreSQL 10.1 libpq and OpenSSL 1.0.2m.
 
 
diff --git a/psycopg/adapter_datetime.c b/psycopg/adapter_datetime.c
index 9d04df40..a78311ee 100644
--- a/psycopg/adapter_datetime.c
+++ b/psycopg/adapter_datetime.c
@@ -100,7 +100,7 @@ _pydatetime_string_delta(pydatetimeObject *self)
 
     char buffer[8];
     int i;
-    int a = obj->microseconds;
+    int a = PyDateTime_DELTA_GET_MICROSECONDS(obj);
 
     for (i=0; i < 6 ; i++) {
         buffer[5-i] = '0' + (a % 10);
@@ -109,7 +109,9 @@ _pydatetime_string_delta(pydatetimeObject *self)
     buffer[6] = '\0';
 
     return Bytes_FromFormat("'%d days %d.%s seconds'::interval",
-                            obj->days, obj->seconds, buffer);
+                            PyDateTime_DELTA_GET_DAYS(obj),
+                            PyDateTime_DELTA_GET_SECONDS(obj),
+                            buffer);
 }
 
 static PyObject *
diff --git a/psycopg/python.h b/psycopg/python.h
index fc8c2fed..fa894bf3 100644
--- a/psycopg/python.h
+++ b/psycopg/python.h
@@ -87,6 +87,7 @@ typedef unsigned long Py_uhash_t;
 #ifndef PyNumber_Int
 #define PyNumber_Int           PyNumber_Long
 #endif
+
 #endif  /* PY_MAJOR_VERSION > 2 */
 
 #if PY_MAJOR_VERSION < 3
@@ -104,6 +105,10 @@ typedef unsigned long Py_uhash_t;
 #define Bytes_ConcatAndDel PyString_ConcatAndDel
 #define _Bytes_Resize _PyString_Resize
 
+#define PyDateTime_DELTA_GET_DAYS(o)         (((PyDateTime_Delta*)o)->days)
+#define PyDateTime_DELTA_GET_SECONDS(o)      (((PyDateTime_Delta*)o)->seconds)
+#define PyDateTime_DELTA_GET_MICROSECONDS(o) (((PyDateTime_Delta*)o)->microseconds)
+
 #else
 
 #define Bytes_Type PyBytes_Type