Temperature and salinity are two of the most important physical factors affect ing estuarine organisms. In niost studies on the effects of physical variables on estuarine organisnus, all factors but one have been rigorously controlled and kept constant. Kinne ( 1971 ) argues that nuultifactorial experiments measuring the effects of two or more environnuental factors acting in concert are necessary to understand the functioning of estuarine organisms under natural conditions. Most previous work on the effects of tenuperature and salinity on osmoregulation of marine and estuarine invertebrates has been done using osmoregulating crusta ceans (Verwey, 1957 ; Lockwood, 1960 ; Dehnel, 1962 ; Todd, 1963 ; Dehnel and Carefoot, 1965 ; Taylor et ci., 1977) . These workers and others have found that most euryhaline invertebrates studied exhibit greater osmoregulatory capabilities in the lower portions of their normal temperature ranges (see Kinne, 1970, 1971, for review). Many workers have investigated the effects of salinity on the blood solute concentrations of various molluscs, but the possible interaction of salinity and temperature on osmotic and ionic regulation in molluscs has not been studied thoroughly. The present study had two objectives : 1) to investigate the inter action of tenuperature and salinity on the osnuotic composition of an osmo conforming marine gastropod Thais haenzastonza and 2) to investigate the effects of long-term salinity fluctuations on this snail. Hemolymph osmolality and con centrations of Na4, K4, Mg24, and Cl under constant and fluctuating salinity were nueasured at 10°,20°, and 30°C. In addition, percent tissue water and the level of ninhydrin positive substances ( NPS ) in the foot tissue were determined.