The activity of black bears (Ursus americanus) in the Great Smoky Mountains National Park was studied through the use of motion sensitive radio-collars. Bears exhibited a tendency toward a crepuscular activity rhythm, although mating activity and changes in the nature and abundance of the food supply modified this pattern seasonally. The most distinct crepuscular rhythm was observed in the spring when expendable energy was limited by a paucity of nutritious food. Bears were more active and more diurnal during the summer when berries were abundant. The level of activity reached a peak during the June–July breeding season. Extensive nocturnal activity was observed only during the fall, and probably was associated with increased foraging in preparation for denning. Activities of females with cubs did not vary seasonally, and this group was active more than any other sex-age group; subadults were active more than solitary adults of their respective sex. Rain, snow, and temperatures above 25°C or below 0°C substantially reduced the level of activity.