Recently, UWB technologies have been Introduced for low rate wireless personal area networks (LR-WPANs). Since the energy level of the UWB impulse radio signals is very low, it is very hard to apply them to a carrier sensing multiple access with collision avoidance (CSMA/CA) scheme, which was adopted as the MAC protocol in IEEE 802.15.4 standard. In this paper, we propose an orthogonal time hopping multiple access (OTHMA) as a multiple access scheme for UWB impulse radio communications, and compare the MAC performance of the proposed scheme with that of CSMA/CA in terms of throughput, success probability, average delay, and user utilization. Simulation results show that the proposed scheme performs better than CSMA/CA as the number of users increases. Therefore, OTHMA can be applied as a multiple access scheme for low power, large scale, and low activity networks in UWB impulse radio communications.