Since 1969 systematic sampling has recorded the appearance in nature of a species, Asellus (Proasellus) coxalis Dollf., and its progressive increase in a biotope (Sarno River, Naples) previously occupied by the single species Asellus (Asellus) aquaticus (L). The two species are at present living in sympatry. It has been noted that both species undergo significant numerical variations in the various months of the year. The maximum peaks for coxalis fall in the two month period April-May and for aquaticus in the two months December-January. Laboratory experiments have confirmed the hypothesis of Haldane (1932), according to whom a rare species is at an advantage in interspecific competition and tends towards a progressive increase. They also confirm the hypothesis of Ayala (1971) who claims that there are divergencies of characteristics in two competing species forced to live in sympatry. Mixed breeding for the study of interspecific competition between the two species, which do not hybridize, was possible thanks to the finding of a distinctive morphological characteristic, the different distribution of «Zenker Cells», which made it possible to identify the two species in vivo, right from the extramarsupial stage.