It is well known that intertidal animals exhibit temporal patterning of behaviour in which locomotor activity occurs at particular times of day and state of tide. Naylor & Atkinson (1976) postulate that such behavioural rhythms are partially controlled by endogenous timing processes which determine the approximate periodicity of a rhythm, as expressed in constant conditions in the laboratory, and that these circadian and circatidal rhythms are entrained by environmental variables which in nature synchronize the expression of behavioural rhythmicity at adaptively appropriate times. Several tidal variables, such as temperature, pressure, light and wave action, have been shown to synchronize and entrain endogenous rhythms (Palmer, 1974; Naylor & Atkinson, 1976) but so far there appears to be little evidence of entrainment of tidal rhythmicity in coastal animals by variations in salinity. A tidal rhythm of responsiveness to water of reduced salinity has been demonstrated by Arnold (1970) in the cirral activity of Balanus balanoides (L.), but entrainment by this response in Balanus has not been tested. In any event the tidal rhythm of cirral activity in Balanus balanoides on the shore is not expressed in barnacles kept in constant conditions in the laboratory (Southward & Crisp, 1965) and therefore appears to be largely controlled by exogenous factors. It therefore seemed worthwhile to investigate the effects of salinity changes upon the locomotor rhythm of Carcinus maenas (L.), the control of which by internal factors (Arechiga, Huberman & Naylor, 1974) and some external factors (Naylor & Atkinson, 1972) is well documented.