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Protocols for mobile ad hoc networks (MANETs) are often designed with the assumption that nodes are equipped with omnidirectional antennas. Spatial reuse in such networks can be significantly improved by using directional antennas, leading to higher system capacity. This gain is associated with a substantial energy saving that results for beamforming the transmitter and/or receiver antennas in the appropriate directions. However, several medium access problems (e.g., hidden terminal, deafness) resurface when directional antennas are integrated into existing MAC protocols. In this paper, we propose a power-controlled MAC protocol for directional antennas that ameliorates many these problems. Our protocol uses separate control and data channels to reduce collisions. It allows for dynamic adjustment of the data-packet transmission power, such that this power is just enough to overcome interference at the receiver. Simulation results demonstrate that the combined gain from using directional antennas and power control results in significant energy saving and improved throughput performance.