The effect of different salinities on the marine snails Hydrobia ulvae and Hydrobia ventrosa (Mollusca: Prosobranchia) was studied in laboratory experiments. For juvenile snails, neither shell growth nor the strength of interspecific competition were affected by salinity within the range of 15–30 psu. In contrast, we found that larvae of H. ulvae were sensitive to salinity: survival was higher and the larvae were more active in higher salinity. The negative influence of low salinity on H. ulvae larvae may not be the only explanation of the complementary field distribution of the two species, since habitat separation is present in areas without spatial salinity variation.