In field populations, it is largely unknown how many mating partners simultaneous hermaphrodites have, although their promiscuous mating is often observed in captivity. Documenting the number of mates in natural populations can expand the understanding of postcopulatory sexual selection and the maintenance of sexual systems. Here we estimated the number of mates in natural populations of the great pond snail Lymnaea stagnalis by estimating the minimum number of fathers within siblings from a given mother. Based on the microsatellite genotypes of mothers and siblings, we found a moderate number of siring fathers, ranging between one and three individuals per clutch. We did not find any offspring produced by self-fertilization, indicating extensive outcrossing. The number of mates was positively correlated with density of potential mates, suggesting that mate availability is an important determinant of their genetic mating success. Although our survey only provides an initial insight into the situation in the field, the results do highlight the potential for postcopulatory sexual selection in natural populations of this simultaneous hermaphrodite.