Systematists may rely on morphometric differences among samples of specimens for the recognition of living and fossil species, even though morphometric differentiation may be caused by non-genetic factors, such as ecophenotypy, differential growth rates and taphonomic mixing. When genetic differences between sexes or among closely related species are expressed as differences in the morphology of the individual or population, potentially valuable information becomes available to the systematist for a variety of genetic and ecological investigations. We have studied the morphology of the freshwater snail Melanaides tuberculata (Muller, 1774) in Israel, where males occur in what would otherwise be normally parthenogenetic (all female) populations. In modern M. tubercalata, sex may be determined by observation of gonadal tissue; in fossil specimens, any classification according to sex must be accomplished using only preservable features of the mineralized shell. Previous research confirmed that in large samples, mean shell shape of male and female snails differed significantly, but the degree of difference was too small to identify the sex of any individual specimen. We apply a three stage process that results with a high degree of accuracy in the discrimination of individual M. tuberculata specimens by sex on the basis of continuous morphological characters: (1) measurement of many aspects of shell morphology of individuals of known sex, and stepwise discrimination to discover which of the variables, if any, contribute to the morphometric differentiation of males from females (one time only, for the species); (2) use of these selected variables in a clustering procedure to make a preliminary assignment of each specimen to sex; (3) use of cluster assignments in a discrimination procedure to optimally predict sex. For species that exhibit morphometric differences between two groups, and for which continuous morphometric variation precludes the a priori recognition of discrete clusters, this sequential procedure may be of broad applicability. These objective methods may be applied to the discrimination within any set of specimens for which the hypothesis of two, and only two, constituent groups may be entertained.