1. By gradually increasing the salinity, Physa heterostropha was made to live quite actively in 25 per cent. sea water. 2. Physa can live but is not active in concentrations between 30 per cent. and 40 per cent. 3. In water stronger than 40 per cent. all the snails died with the exception of the Davies Lake lot, which had retreated within their shells, but which revived when placed in 5 per cent. sea water. 4. Whether the 5 per cent. increase in concentration was made at intervals of one or two days, or even a week, seemed to make no difference in the ability of the snails to become acclimated to the sea water. 5. All the races survived a sudden change to 5 per cent., 10 per cent., 15 per cent., 20 per cent. and 25 per cent., but the Davies Lake stock was the only one active in 20 per cent. and 25 per cent., at the end of one week. 6. The snails from Davies Lake may be regarded as a stock which is more hardy to sea water than the other stocks used in the experiments.