We estimated outcrossing rates and correlated matings in a natural population of the hermaphroditic freshwater snail Ancylus fluviatilis, using progeny arrays and polymorphic allozyme markers. This design circumvents drawbacks of earlier studies, and yields maximum likelihood estimates of outcrossing rates at both the individual and population level. Adult snails were sampled during the spring breeding period, allowing for prior copulations and sperm transfer under natural conditions. Once in the laboratory, they were kept isolated for 21 days and laid egg capsules, and emerging hatchlings from 42 families were scored for five polymorphic loci. 87 of 848 offspring exhibited non-maternal alleles, representing a minimum population-level outcrossing of 10.3%. Maximum likelihood estimates are in the range 13-15%. Importantly, there is significant among-family heterogeneity, as only 16 of 42 families are estimated to have outcrossed, in accord with high estimates of correlated selfing within progeny arrays. Moreover, the high proportion of full sibs among outcrossed sibs suggests a limited extent of multiple paternity. Our data reveal important heterogeneity in the mating system of an animal hermaphrodite, and indicate the potential for evolutionary change in the breeding system.