In the Hercynian Massif, a geologically old area in Southern France, various morphologically distinct populations of the Gammarus pulex-group were found. When applying the morphological criteria postulated by Karaman & Pinkster (1977), at least three species can be distinguished. However, in hybridization experiments these populations proved to be interfertile, and thus conspecific. Likewise it was proved that this species cannot be identical with G. fossarum or G. ibericus, it is considered a new species: G. stupendus. Because of its polymorphic nature, it is difficult to describe the species and to find discriminating characters with G. fossarum and G. ibericus. The distribution of the new species and of G. ibericus are discussed.