A prioritized medium access control (PMAC) protocol is proposed for wireless sensor networks, based on a binary count down approach. PMAC effectively eliminates data collision and optimizes channel allocation. Moreover, as many networks nowadays compose traffic with several priorities, the simple yet effective design of PMAC offers strict service differentiation for prioritized packets. In this paper, PMAC is evaluated by collision probability, reneging/dropping probability, network throughput and channel efficiency. Our simulations show that the protocol effectively supports traffic differentiation and achieves very low packet dropping (both reneging and blocking) probabilities under the channel capacity. When the network is overloaded, it still maintains extremely stable and high channel throughput. It is demonstrated that PMAC performs superior in multihop networks, further proving the advantages of the proposed protocol. Featured with efficiency, simplicity, scalability, fairness, mobility and QoS provisioning, PMAC may be well served for the next-generation wireless networks.