The activity of eight male striped shore crabsPachygrapsus crassipeswas recorded for 33 days in a LD 12:12 cycle with light on from 6:00 h to 18:00 h. All eight crabs displayed an overt semi‐diurnal tidal rhythm for the entire recording period, with peak activity occurring two hours after each high tide. In all crabs a 24‐h activity component was also expressed, with activity reaching a maximum 2–5 h after light‐off. This activity pattern was presumed to be of adaptive significance, allowing the organisms to remain active longer during the night hours in search for food when uncovered by the ebbing tide than in the daytime when uncovered due to predatory pressure. It was noted that while the tidal rhythm displayed an average 12.4‐h periodicity, there appeared to be a correlation between activity and the precise time of local high tide which occurred at times as little as 20 min and as much as two hours later each solar day. Two hypotheses regarding the underlying clock timing the observed rhythms were discussed, that of an internal oscillatory mechanism and, second, a response of the organism to pervasive subtle geophysical cycles.