Arduino Mega 2560 & Xbee

Dear,
with or without shield xbee the same code : no problem on UNO; no TX/RX on the Mega !? ; and this is strange because the sender indicates receiving an ACK …
Without shield : XBee <–> Mega: [DIN toTX0] & [DOUT to RX0], Vcc and GND.

#include <XBee.h>

XBee xbee = XBee();
ZBRxResponse zbRx = ZBRxResponse();
           
void setup () {
   Serial.begin(9600);
   xbee.setSerial(Serial);
 Serial.println("starting up yo!");
}

void loop () {
   Xbee_rx();
}

void Xbee_rx(){
 xbee.readPacket();
 if (xbee.getResponse().isAvailable()) {
   if (xbee.getResponse().getApiId() == ZB_RX_RESPONSE) {

     xbee.getResponse().getZBRxResponse(zbRx);
 Serial.println("Got an rx packet!");
 if (zbRx.getOption() == ZB_PACKET_ACKNOWLEDGED) {
             Serial.println("packet acknowledged");                   
                } else {
                       Serial.println("packet not acknowledged");
                        }
     for (int i = 0; i < zbRx.getDataLength(); i++) {
            Serial.print("payload [");
            Serial.print(i, DEC);
            Serial.print("] is ");
             Serial.println(zbRx.getData(i));
            }                           
   }
 }
}

I See, here, some peoples using SoftwareSerial.h by it’s not my way, for the moment. Xbee used is ZB S2C Pro, and Mega 2560 (Clone).
Philippe.