Experiments were conducted to determine the survivability of Littorina irrorata and L. lineolata under various conditions of water temperature and salinity, and to determine the sodium and potassium ion concentrations in the haemolymph of L. irrorata under various conditions of water temperature and salinity. The vertical movements of L. irrorata in the field and under laboratory conditions were studied. It was found that L. lineolata survived heat better than L. irrorata, but L. irrorata survived cold better than L. lineolata. L. lineolata survived the higher salinities better than L. irrorata. L. irrorata was more resistant to desiccation than was L. lineolata. The low salinities (0°/oo, 5°/oo) placed more stress on both species than did the highest salinity (40°/oo). These observations were related to the ecologies of the animals. L. irrorata was found to maintain sodium and potassium ions in the blood at higher concentrations than those in the surrounding water, at all temperatures. This apparent control was not due to the method of blood extraction, to inaccuracies in glassware calibration, or to lack of contact with the medium. However, the control appeared to depend in part on the ability to retract into the shell from time to time. Temperature extremes (0°C., 42.5°C.) did decrease the snail's ability to maintain the difference in Na+ and K+ concentrations. The vertical movements of L. irrorata in nature correspond with the tidal movements. The cue for the snails' movements is unknown. The movements are not cued by humidity variations, time alone (inherent rhythm), the direct effects of sun and moon, or immersion and exposure. It is suspected that the vascular characteristics of the marsh grass on which L. irrorata lives are the cue for the snails' movements.