Activity onsets of horn sharks, Heterodontus francisci, were studied in the natural environment at Santa Catalina Island, California. The mean of activity onsets of these nocturnal sharks occurred 76 min after sunset which corresponded to an approximate light level of 0.03 lux. During laboratory studies under a fixed LD 12:12 (12 hr light- 12 hr dark) regime, the L phase of which was 8 lux, H. francisci showed anticipation of the D phase. Under 23 days of constant dim light, H. francisci exhibited a drifting circadian rhythm with period lengths, for one individual, of 20.8 hr under 2.0 lux, and 23.0 hr under 0.13 lux. Under 15 days of constant darkness, the sharks showed a high level of aperiodic activity. During laboratory experimental dusk transitions, the light intensity eliciting onset of activity was: 1), proportional to the amount of time sharks spent in the L phase of the LD regime, and 2), independent of the level of Hght duiing the L phase of the LD regime.