Osmoregulation of the shore crab (Carcinus maenas L.)Regulation curves dealing with osmotic concentration, conductivity and chloride concentration of the haemolymph have been obtained for Carcinus maenas. Between about 18 and 30°/oo S the quantities mentioned above are maintained at a more or less constant level (regulation), at higher salinities conformity is found, at lower salinities a weak regulation exists. There is a distinct temperature effect, the quantities are correlated negatively with the medium temperature, moreover, a greater regulation capacity at higher temperatures is shown; the curve slope angle is smaller. Combined with the temperature effect the regulation curves show a relation with the migration pattern of Carcinus. Comparison with literature data suggests geographical races. The urine production has been measured at salinities below 25°/oo S. Salinity and frequency of urine production show a negative correlation; in addition, a positive temperature influence on urine production is found.