Reproductive success is a crucial variable to understand the action of sexual selection, but its quantification is not straightforward because several factors cause nonrandom fertilization success. One of these factors is female sperm storage, which leads to prolonged paternity gain (paternity longevity). In particular, knowledge about paternity longevity is crucial in simultaneous hermaphrodites, since their ability to self-fertilize adds a further layer of complexity to the quantification of male reproductive success. Here, we investigated the pattern of outcrossing over time after a single mating in a freshwater snail, Lymnaea stagnalis, using a microsatellite marker. We found that offspring are produced from stored sperm for a little over 2 months on average. Furthermore, the pattern of paternity indicates an active role for the female reproductive system in the transport of received sperm to the sperm storage organ. To quantify paternity longevity in a sperm-storing mating system, we propose to use the time when half the offspring are sired by a focal donor (Paternity Longevity 50; PL50), in addition to total storage duration. Intriguingly, we reveal that overall body size is positively correlated with paternity longevity. Although the exact mechanism of sperm storage and its contribution to male reproductive success remain to be revealed, this study provides useful information and new perspectives on sperm storage and sexual selection in this simultaneous hermaphrodite. © 2014 © The Author 2014. Published by Oxford University Press on behalf of The Malacological Society of London, all rights reserved.