(1) The asteroid Luidia clathrata becomes inactive when transferred from 27‰ sea water to l6‰, but recovers its activity within 48 hours. (2) The volume of intact animals increases with the initiation of the hyposmotic stress, but returns to somewhat above the original level within several days. This suggests regulation of the volume of the coelomic fluid. (3) The changes in the per cent hydration and level of ninhydrin positive substances in the podia and pyloric caeca of hyposmotically stressed animals indicate that intracellular isosmotic regulation occurs. (4) The increase in the rate of ammonia excretion of hyposmotically stressed animals suggests that the decrease in tissue NPS levels results from their catabolism. (5)Luidia clathrata is a euryhaline echinoderm, responding to reduced salinities by coelomic fluid volume regulation and isosmotic intracellular regulation.