A previous study hypothesized that parthenogenesis arose spontaneously in the freshwater prosobranch snail, Campeloma decisum, and was subsequently maintained by strong sterility selection against males caused by a digenetic trematode, Leucochioridiomorpha constantiae. The present study examines whether parthenogenesis arose spontaneously or by hybridization between genetically distinct sexual ancestors. Genetic variation was examined at 19 enzyme loci in 11 parthenogenetic and seven sexual populations. Parthenogens from North Carolina and Wisconsin are homozygous at all loci, which suggests that parthenogenesis arose spontaneously in the absence of hybridization. Parthenogens from other northern populations are heterozygous at numerous loci and these individuals have gene combinations found in distinct sexual lineages. These findings suggest that parthenogenesis arose both spontaneously, followed by strong selection of these families from parasitic castration of males, and by hybridization. Low levels of clonal diversity occurred within and among parthenogenetic populations and clonal diversity is derived from patterns of genetic diversity present in sexual populations. Lastly, fixed allelic differences between southeastern and southern sexual individuals suggest that these areas are inhabited by cryptic species.