Background. Sexual conflicts between mating partners can strongly impact the evolutionary trajectories of species. This impact is determined by the balance between the costs and benefits of mating. However, due to sex-specific costs it is unclear how costs compare between males and females. Simultaneous hermaphrodites offer a unique opportunity to determine such costs, since both genders are expressed concurrently. By limiting copulation of focal individuals in pairs of pond snails (Lymnaea stagnalis) to either the male role or the female role, we were able to compare the fecundity of single sex individuals with paired hermaphrodites and non-copulants. Additionally, we examined the investment in sperm and seminal fluid of donors towards feminized snails and hermaphrodites. Results. Compared to non-mating focal snails, reciprocating individuals as well as male and female copulants experienced a significant fecundity reduction (∼40%) after, on average, 3.07 0.12 copulations in their allowed roles (for donors 2.98 0.16 copulations and for recipients 3.14 0.12 copulations). In a single copulation, significantly more sperm was donated to partners that were restricted to mating in the female role than to hermaphrodites, while seminal fluid transfer was unaffected by recipient type. Conclusions. Our data indicate that the costs of mating in both sex functions are high in L. stagnalis. This conclusion is based on fecundity data collected separately for male and female copulants. Male mating costs result from investment in expensive ejaculates, composed of sperm and seminal fluid. For female copulants, fecundity reduction correlated with transferred sperm numbers in the first copulation, while differences in transferred quantities of seminal fluid were not detected. These findings may point toward a "sperm effect" as a novel feature of pond snail reproductive ecology. In conclusion, sex allocation and sexual conflict both contribute to decreased female fecundity in pond snails.