Asiatic black bears (Ursus thibetanus) are threatened by habitat loss and poaching, especially in the tropical portions of their range; reserves serve a crucial role in conserving this species. Yet data on spatial and habitat requirements for this species in tropical areas, necessary for assessing the efficacy of reserves, is virtually nonexistent. We used mainly ground-based telemetry to investigate home range sizes of the endangered Formosan subspecies (U. t. formosanus) in the largest park in Taiwan. The largest observed home range (117 km²) was an adult female with a satellite radiocollar. Normally, male bears have significantly larger home ranges, but males tracked with ground telemetry often could not be located due to the rugged terrain and limited accessibility of our study area, so their home ranges were underestimated. This is a common, but often neglected problem of telemetry studies in protected areas with difficult human access. Although elevations ranged from 300 to >3,500 m, bears mainly used areas below 2,000 m, selecting broadleaved and mixed broadleaved–coniferous forests. Production of acorns (Cyclobalanopsis and Quercus), a sought-after fall food, varied yearly. One site in the interior of the park produced an abundance of acorns in some years, attracting a dense congregation of bears; however, females and subadult males were socially excluded. Despite limitations of our telemetry data, we observed that half the bears, all caught near the center of the park, traveled beyond the boundaries where they were more vulnerable to illegal hunting, suggesting that more protection is needed along the edges of the park.