In addition to the well-known freshwater occurrence of Gammarus duebeni in the inland of Ireland, a similar inland occurrence in the western part of Brittany (= Bretagne, France) is demonstrated. In Brittany, as in Ireland, G. duebeni occurs in waters with low sodium concentrations (often less than 23 mg/l), whereas elsewhere in its range, G. duebeni lives in waters with a raised sodium content (either in mixohaline waters, or in “fresh” waters loaded with salts through gales from sea). Reid has proposed the designation G. duebeni α for the Irish freshwater form, G. duebeni ß for the brackish water form. The subdivision of G. duebeni is strongly reinforced by the findings of Sutcliffe et al., indicating physiological differences between the α and ß form. In the present paper, it is shown that significant morphometrical differences exist between the two forms, especially in the length/width ratio of the merus of the fifth leg. A mathematical test, the coefficient of difference, for this character shows it is around or above the standard of subspecific difference. On the other hand, all possible crosses between limnic animals from Eire and Brittany, and mixohaline animals from northern France and Holland, proved to be fertile. The available evidence (spatial isolation through different habitat, important physiological and morphological differences, interfertility) points very obviously in the direction of two different subspecies: the one, the nominate subspecies, living in waters with rised ion concentrations (restricted neotype locality, supralittoral rockpools at Stora Kalsöy, near Bergen, Norway); the other, celticus Stock & Pinkster, 1970, living in waters with low ion — in particular sodium — concentrations (type locality Lough Corrib, Eire). A detailed examination of the distribution pattern of G. duebeni celticus, and competing gammarids like G. pulex and Echinogammarus berilloni, in the inland of Brittany (Bretagne), makes it probable, that G. duebeni was the oldest freshwater species in that area (presumably originating from mixohaline or marine ancestors in one of the earlier interglacial periods), and that G. pulex and E. berilloni were later, postglacial, invaders in that part of Europe.