In this letter, a new energy-efficient medium access control (MAC) protocol applicable to IEEE 802.11 is suggested. The proposed adaptive beacon listening (ABL) protocol dynamically determines the beacon listening interval (BLI) of mobile station based on the pdf of the estimated round-trip time (RTT). Theoretical and simulation results for power efficiency and average delay are derived. In general, there is a trade-off between delay and power conservation, in that a small number of beacon listenings save power, while a large number of beacon listenings reduce delay. However, the proposed scheme shows that we can achieve better performance with respect to average delay and power efficiency.