1. Sodium influx was examined in Gammarus duebeni from freshwater habitats on the Kintyre and Stranraer peninsulas in western Britain, and from a brackish-water habitat in Ireland. The affinity for sodium ions in the uptake mechanism at the body surface was similar in animals from the three localities.
2. Compared with the parent population from Kintyre, an experimental population established for 2 years in water with a lower sodium concentration showed an increased affinity for sodium.
3. Sodium losses in the urine of animals from the above localities were negligible at external salinities below about 2% sea water. In contrast, urinary sodium losses in animals from a brackish-water population in Britain were higher at salinities ranging from 40% sea water to well below 2% sea water.
4. The affinity for sodium ions in uptake mechanisms at the body surface and in the antennary glands of G. duebeni from a wide range of habitats shows a market correlation with the sodium concentration of the habitat. The permeability of the body surface to outward movement of sodium is similar in G. duebeni from brackishwater and freshwater habitats.
5. It is suggested that most of the observed physiological differences between populations of G. duebeni are phenotypic in origin. The status of the freshwater ‘race’ in Ireland is briefly discussed.