1. The salinity reactions of Asellus aquaticus from fresh and Baltic brackish water (6‰), of Idotea baltica and of Gammarus spp. from brackish water were studied with the apparatus of Hodgson (1951). Both natural brackish-water and pure NaCl solutions were used. 2. None of the experimental animals had, for brackish water, reaction thresholds low enough to allow behavioral selection of salinities to occur in natural conditions. 3. The reaction threshold for NaCl solutions was higher in brackish-water than in fresh-water specimens of Asellus. Asellus always preferred the more dilute concentration. Idotea baltica and Gammarus preferred brackish water to fresh water. Gammarus preferred even 6‰ NaCl to fresh water. 4. In Asellus, the chemoreceptors mediating the response to salinity variations seem, for the most part, to be situated on the antennae and/or antennulae. In Gammarus, the salinity receptors are situated on the antennae and antennulae.