Activity rhythms in groups of captive beach beetles (Chaerodes trachyscelides White) have been recorded in an actograph over a period of 29 days. Under constant illumination and when no sand was provided for the beetles to burrow in abnormal behaviour occurred. With sand and in constant darkness a strictly nocturnal activity period with apparently circaseptan components superimposed was observed. The beetles in their natural habitat are confined to the debris zone where they feed on washed up seaweed and this debris zone moves up and down depending on the heights of the tides. Circaseptan elements in the beetles' activity may act as adaptations to the weekly alternations between spring- and neap-tides.