In the lower intertidal zones of the river Crouch there is a wide salinity fluctuation from a minimum when they are just covered by the tide to a maximum at full tide. Isohalines of minimal salinity rise to higher shore levels as the estuary is ascended. Marine animals, therefore, restricted to life above a particular isohaline, are found higher in the intertidal zone in the upper reaches of the estuary than in the lower. This situation applies to Hydrobia ulvae which, in addition, is limited in its distribution in the higher levels of the intertidal zone by exposure to air. Since the tidal rise and fall is less in the upper reaches o f the estuary, the animals are restricted to a narrowing belt between the converging isohaline a nd high water mark. Within this belt, however, passive stranding controls the distribution o the animals.Experimental evidence suggests that the upstream distribution of H. ulvae is controlled primarily by the effects of salinity upon the activity of the animals in general and upon the floating response in particular. Reduced activity restricts the number of animals which float and floating is eliminated in low salinities. These influences result in the population occurring as far as the upper tidal limit of the estuary and being confined within salinity extremes which the animals can tolerate physiologically.