In this paper we present an energy efficient priority based MAC protocol for Wireless Sensor Networks (WSNs). We abbreviate it as PRIMA. The PRIMA protocol consists of two phases; a clustering phase and a channel access phase. Clustering the sensor network makes the MAC protocol to handle well network scalability issues. The channel access is composed of a hybrid mode of TDMA and CSMA. CSMA mode is used to communicate control messages, while data messages are assigned TDMA slots. Doing so, minimizes packet collisions and consequently minimizing energy consumption. Our PRIMA protocol forces the nodes that have no data to send to go early into a sleep state to save energy, this minimizes the idle listening periods which is considered as a main source of the energy consumption in sensor networks. The PRIMA protocol provides Quality of Service (QoS) by employing a queueing model to classify the traffic depending on its importance into four different queues. Higher priority queues have absolute preferential treatment over low priority queues. Through simulations and analytical analysis, we evaluate the performance of our proposed MAC protocol and compare it against the Q-MAC protocol. Results have shown that our protocol outperforms Q-MAC in terms of energy consumption, packet delivery ratio and average packet delay.