An ultrasound tracking system is an inexpensive and accurate technique for indoor object tracking. In this paper, we describe an IEEE 802.15.4-based active tracking system that tracks multiple mobile objects effectively. We analyze the problems of the active tracking system with multiple objects and propose an adaptive beaconing algorithm as the solution. In our algorithm, mobile nodes overhear the beacon messages of other mobile nodes and adaptively adjust their beaconing periods to avoid a simultaneous ultrasound pulse. We implemented the tracking system in a real environment and measured the performance of the system. The experimental results show that our algorithm successfully works with multiple mobile nodes and improves performance on the success rate.