Behavioural responses of the colour forms of Carcinus maenas to salinity variation were investigated, and related to their physiology and distribution in an estuary. Red males, characterised by a thicker carapace were unable to survive in as low salinities as green males; this was reflected in their poorer osmoregulatory capabilities. Haemolymph osmolality and ion concentrations of red crabs decreased at a faster rate and reached lower levels than in green crabs. Haemolymph osmolality and choice behaviour did not vary with size. In the tidally mixed estuary male and female crabs occurred in roughly similar proportions. Most were green and generally smaller than their open shore counterparts. Migration out of the estuary in winter was reversed in late spring. Differences in salinity tolerance of red and green crabs were reflected in salinity preference behaviour. Green crabs persisted longer in the lowest range of salinities tested, especially if a shelter was available. Prior acclimation affected the timing of choice behaviour; the lower the salinity of acclimation the faster the time of exit from the lowest range of salinities tested, and vice versa. Estuarine green crabs exhibited endogenous locomotor activity of circatidal periodicity and were less responsive to episodes of low salinity than open shore red and green crabs. Constant low salinity initiated a rhythm of circatidal periodicity in arhythmic red and green crabs; red crabs reacted faster and were more active upon salinity change than green crabs. The amount of locomotor activity induced after prior acclimation was similar within each acclimation salinity tested. Carcinus detected salinity variation by responding to the concentrations of Na and Cl in seawater, and was able to differentiate between salinities separated by as little as 0.5ppt., - General physiological changes appear to occur before behavioural responses are mediated; they probably act as cues for the behavioural responses, which appear not to be triggered by specific receptors. Behavioural and physiological responses combing to enhance, the survivability of crabs in changing salinities.