As far as I know, Potamopyrgus has been included among either the Hydrobiinae (Thiele, 1929: 141; Climo, 1977) or the Littoridininae (Wenz, 1939; Starobogatov, 1970). Taylor (1966: 184) discussed its position but did not decide where it really should be classified. During the last few years the Hydrobiinae and the Littoridininae have been defined in a detailed and excellent manner by Taylor (1966) and Davis, Mazurkiewicz & Mandracchia (1982). Davis et al. gave a table comparing the Hydrobiinae and the Littoridininae and two additional subfamilies involving 14 characters. Table 1 repeats the one given except that the additional two subfamilies (Lithoglyphinae and Nymphophilinae) have been omitted and three additional characters and the new subfamily Potamopyrginae added. This new table gives evidence why authors have been uncertain as regards Potamopyrgus. The Potamopyrginae differ from the Hydrobiinae and the Littoridininae by character 11b (central tooth), from the Hydrobiinae in addition by characters 5b, 6b, 8c and 13b, and from the Littoridininae by characters 1a, 2a, 3a, 6b, 8c, 12b and 14a. Therefore, the introduction of a new subfamily for Potamopyrgus is justified; this new subfamily adequately reflects the peculiarities of Potamopyrgus.