A survaey of Corsican Molluscs, in October 1980, confirms the extension of Potamopyrgus jenkinsi. However two types of biotopes could be distinguished. In the first Potamopyrgus is very abundant and Isidora truncata (= Bulinus truncatus) was not found. In the second type Potamopyrgus are less numerous and coexist with Bulinids. That the colonization by Potamopyrgus is relatively ancient leads us to believe that the second type does not correspond to a preliminary stage of contamination, but rather reflects a stage of equilibrium.