The local movements and activity patterns of American lobsters, Homarus americanus, were monitored inside a 50 m by 50 m underwater enclosure (mesocosm) using ultrasonic telemetry. Forty-four lobsters of both sexes, ranging in size from 62 to 93 mm in carapace length, were continuously tracked for 2-10 days in 2002 and 2003. As a population, the movement rate of lobsters depended on time of day, as defined by dawn, day, dusk or night. Lobster movement rates were significantly higher during night and dawn than day and dusk hours. Movement rates did not differ by lobster sex, size or between years of the study. The effect of time of day differed between lobsters, and there was considerable variability in the time of day when individual lobsters were most active. Thirty lobsters moved significantly more during the night, five moved significantly more during the day, and nine did not move significantly more during the day or night. Therefore, while there was a general tendency for lobsters in this study to be more active at night, certain factors in their natural habitat modulated this nocturnal bias, which led to a tremendous amount of variability in their daily patterns of behavior.