Shifts in activity patterns and time budgets in response to environmental variables may yield insights into key factors affecting animal life histories. Activity data on wild Asiatic black bears Ursus thibetanus are limited, and non-existent in tropical portions of their geographic range. During November 1998–November 2000, we monitored the activity (active vs. inactive) of bears within a national park in the tropics of central Taiwan. On the basis of 4630 readings from motion-sensitive transmitters obtained in 24-h monitoring sessions, bears were active 54–57% of the time over the course of the year. However, bears were more active during summer (60±15%sd) and fall (60±15%) than in spring (47±13%). They were active mainly during daylight hours in spring and summer, and also during fall when acorns, a principal fall food, were scarce. However, extensive nocturnal activity occurred in fall when acorns were abundant. Increased nocturnal activity in fall is consistent with studies of the related American black bear Ursus americanus, and suggests that this behavior is not necessarily driven by preparation for winter denning or because of disturbance by hunters (as posited in previous studies), as bears in our study did not den (except parturient females) and were not hunted or otherwise disturbed by people.