Radio Frequency IDentification (RFID) is a technology used on a wide set of applications. One of its main advantages is its ability of identifying objects in a wireless fashion with no contact or a direct sight line among the communicating devices. By improving the underlying collision resolution protocol in an RFID system, the power consumption of such devices is improved as well. In this paper, we present a new p-persistent CSMA collision resolution protocol for RFID systems. We compare our protocol with a non-persistent CSMA protocol previously proposed in the literature as well as with the ISO18000-7 standard. By measuring the average identification time for a wide range of tags, we show by simulation that our protocol outperforms both protocols.