Minnow predation by 10 young American alligators ( Alligator mississippiensis ) was systematically measured during four daily time periods under four different conditions of lighting in order to investigate a circadian rhythm of redatory behavior.The four daily time periods were night (1:00 a.m. -7.00 a.m.),morning (7.00 a.m.- 1:00 p.m.), afternoon (1:00 p.m.-7:00 p.m.), and evening (7:00p.m. -1:00 a.m.). Each of the following lighting conditions had a duration of 4 weeks:continuous complete darkness (DD); continuous artificial illumination (LL); naturally varying Ught-dark conditions (natural LD); and reversed light-dark conditions with artificial lights on at sunset and off at sunrise (reversed LD). Predatory behavior (i.e., the number of prey fish consumed wholly or partially during each test session) varied significantly as a function of the interaction between time period and lighting condition.Under natural LD, the mean number of prey killed during night sessions was significantly higher than either morning or afternoon sessions. Under reversed LD,the pattern of predation reversed from that exhibited under normal Ughting, with both morning and afternoon predation significantly higher than either evening or night. Under conditions of continuous illumination (LL and DD) the natural LD circadian pattern persisted for over 1 week with significantly higher predation rates during the night periods as ompared to the morning and afternoon periods. The gradual transition of predation pattern in response to manipulations of the light-dark cycle suggests that the circadian rhythm of alligator predadon is dependent upon light-dark variation for entrainment.