Morphologically distinct populations of the Gammarus pulex group which should be attributed to at least three species applying morphological criteria postulated by Karaman & Pinkster (1977) have been reported in the Massif de la Sainte Baume (southern France). These populations proved to be completely interfertile and thus were considered conspecific. This group of populations, in which three major forms A, B and C were distinguished, has been described as Gammarus stupendus Pinkster, 1983. Systematic sampling was carried out to determine the distribution of the various forms of G. stupendus in the Massif de la Sainte Baume and adjacent areas, and the relation between morphological and genetic differentiation has been investigated. Genetic differentiation has been studied in four polymorphic gene loci: Gpi, Got-1, Mdh-1, and Mdh-2. As the populations used in hybridization experiments by Pinkster (1983) proved to be genetically very similar, additional crossbreeding experiments have been carried out with two genetically more distinct populations. Moreover, experiments were run to test the stability of diagnostic characters in each form A, B and C of G. stupendus. No clear correlation could be found between any of the factors morphology, genetic differentiation, and geographic distribution pattern. In the crossbreeding experiments genetically more distinct populations appeared to be incompletely interfertile. Thus, the various G. stupendus populations, exhibiting different degrees of interfertility which may ultimately result in reproductive isolation, may indicate different stages in the process of speciation.