1. Osmotic balance was studied in Hydrobia ulvae and Potamopyrgus jenkinsi over the range 100% sea water to fresh water, by determining the freezing-point depression of the urine in the different solutions. 2. Hydrobia ulvae was slightly hyperosmotic from 100 to 50% sea water, and sometimes initially markedly hyperosmotic in 25% sea water at 5°C. The urine was always markedly hyperosmotic relative to fresh water, and the animals were withdrawn. Experiments with phenol red indicated that the tissues were not shut off from the medium. 3. Potamopyrgus jenkinsi was hyperosmotic from fresh water to 100% sea water. Osmotic balance in fresh water is maintained in part by the excretion of a urine hypo-osmotic relative to the blood. 4. There was some variation in the reaction of different ecological groups of Hydrobia ulvae and Potamopyrgus jenkinsi to the experimental conditions. 5. In the Hydrobiidae, whether transformed from a lower to a higher salinity or vice versa, survival outside the viable range was longer at 5°C. than at 15°C., although, within the range, activity occurred more rapidly at 15°C. 6. In fresh water, winter animals of both species had a higher osmotic concentration of the urine than summer animals, but no differences in osmotic concentration correlated with temperature were demonstrated.