This study was initiated to determine the immunologic specificity of snail hemolymph antigens and to evaluate their serotaxonomic usefulness. Hemolymph antigens were obtained from snails of the families Planorbidae and Lymnaeidae, representing 9 genera, 17 species, and 26 strains. The interfacial ring test (IRT), and a gel-diffusion system, were employed to compare the precipitating activity of the various antigens in the presence of specific snail antisera. Antigenic differences between genera were demonstrable in the IRT. Generic specific precipitin reactions were obtained following absorption, from the antisera, of familial cross-reacting antibodies. Congeneric species or strains of a species could not be differentiated by the IRT. Gel-diffusion studies confirmed the results of the IRT with respect to generic differences; in addition, the gel-diffusion technique proved sensitive enough to demonstrate, with some antisera, antigenic differences between congeneric species. However, strains of a species were not differentiated. The results of this study indicate that, in spite of certain limitations, serologic techniques may serve as valuable adjuncts to the methods now employed by the molluscan taxonomist. For example, present serologic evidence suggests that species assigned to the nominal genera Australorbis, Tropicorbis, and Biomphalaria are congeneric-a concept recently set forth by several malacologists on the basis of anatomical studies. Furthermore, serologic evidence indicates that snails now assigned to the genus Lymnaea may, in fact, to more than one genus. Requisite to control and to all biological studies on the snail hosts of schistosomes is a foundation of sound taxonomy. The present taxonomic status of many of the snails of medical importance has not changed appreciably since 1954 when it was noted that Confusion was found to exist not only in the nomenclature of the species, but even in the delimitation of the genera and the family to which the snails belong (Anonymous, 1954). Malacologists first used shell characters to establish species of snails. They failed, however, to appreciate individual variation or the plasticity of these animals in response to environmental challenges, and consequently the literature became flooded with a mass of specific names. For example, although 200 or more specific names have been ascribed to the South American Planorbidae (Harry, 1962), it is doubtful whether more than 25 valid species exist, of which a half dozen or so may serve as snail-hosts for Schistosoma mansoni. A similar profusion of synonyms exists for the African Planorbidae. Received for publication 18 September 1965. * These studies were supported, in part, by Grants AI-00513 and 5TI A-I 46 from the NIAID, NIH, U. S. Public Health Service, Bethesda, Maryland. t Portions of this paper were presented before the 2nd European Malacological Congress, Copenhagen, Denmark, on 11 August 1965. Recent applications of morphologic criteria, principally those associated with snail genitalia, have improved immeasurably our taxonomic concepts; however, even these criteria have failed to establish universally recognized genera and species. A system of classification can be no better than the criteria upon which it is based. Having reached an impasse with available criteria, the need for newer approaches for differentiating snail taxa is apparent. Some progress has been made in this direction by the application to molluscan taxonomy of chromatography (Kirk et al., 1954; Wright, 1959; Wright and Claugher, 1959), electrophoresis (Targett, 1962, 1963; Wright and Ross, 1963), and by the enumeration of snail karyotypes (Burch, 1960, 1961, 1964). Taxonomic problems associated with various plant and animal groups have been resolved, with considerable success, by means of serologic and immunologic techniques (Leone, 1964). However, except for the early studies of Erhardt (1931) and Wilhelmi (1944), who attempted to elucidate the phylogenetic relationships between Mollusca and other phyla by immunologic methods, and the immunoelectrophoretic study of Tran van Ky et al. (1962), in which the total proteins of four snail species were compared, malacologists have rarely employed these techniques. In the present study an attempt has been made to determine the serologic specificity of