diff --git a/cores/arduino/HardwareSerial.h b/cores/arduino/HardwareSerial.h
index 59f0584..11e8f6c 100644
--- a/cores/arduino/HardwareSerial.h
+++ b/cores/arduino/HardwareSerial.h
@@ -134,4 +134,14 @@ extern HardwareSerial Serial4;
 extern HardwareSerial Serial5;
 #endif
 
+#if SERIAL_6_ENABLE
+extern HardwareSerial Serial6;
+#endif
+#if SERIAL_7_ENABLE
+extern HardwareSerial Serial7;
+#endif
+#if SERIAL_8_ENABLE
+extern HardwareSerial Serial8;
+#endif
+
 #endif