Gammarus oceanicus is nearly omnipresent and coexists with G. locusta, G. salinus and G. zaddachi. The 3 latter spp. show migrations between habitat types in tight correlation with their breeding periods. G. duebeni is mainly found in rockpools. This behavior results in a complete separation of the breeding species ppoulations. These findings are discussed in relation to minimum requirements for coexistence, i.e., displaced breeding periods and non-overlapping size distributions, and to mating competition through the formation of interspecific precopulas.