Underwater observations were made on the diel activity patterns of yellow perch (Perca flavescens) in Cazenovia Lake, Madison County, New York (42°56′N, 75°52′W). Perch underwent a characteristic progression of activities during dusk at relatively predictable light levels and times. Patterns within the progression included increased swimming, last feed, group breakup, slowdown, and final stop. Activity at sunrise was the reverse of the sunset sequence except that some activities occurred at lower light levels than did the analagous activities in the evening. Differences exist in the twilight activity patterns of perch in different lakes; within a lake, both ontogenetic and subpopulation differences are also observed. These differences may be responses to ecological conditions, such as relative predation levels, or they may reflect the extreme plasticity of the species.Key words: behavioral ecology, changeover, diel, diurnal, fish, light, nocturnal, ontogeny, plasticity, temperate lake