Continuous recordings of the locomotor activity of the intertidal grapsid crabHelice crassain constant laboratory conditions were obtained from two individuals for periods in excess of three months. The records show that the strength of the tide‐related rhythm varies with time. On occasion the crabs are arrhythmic or only weakly rhythmic but clear rhythms are often spontaneously re‐established. Even after 3–6 months in the laboratory, clear rhythmicity is seen at times. Rhythms with periods varying from 6.5 h to 28 h were recorded. The relationship between the period of the overt rhythm and that of the underlying oscillatory system is considered.