Monogamy is a mating system with pairs made of only one male and only one female. But social monogamy cannot inevitably imply the sexual exclusivity between mates, which is yet the case in genetic monogamy on which the offspring results only from the observed pair. The genetic monogamy is today considered as an exception in monogamous animals but the possibility to store sperm for females in some monogamous species makes cooccurrence of genetic monogamy with extensive mate changes possible. Schistosome is a socially monogamous endoparasite with female living inside the gynecophoric canal of its male. If mate changes have been demonstrated in this parasite, resulting offspring needs to be proven, especially if we consider the presence of a receptaculum seminis in female schistosomes. In this paper, we show the first evidence of rapid offspring production resulting from mate change in a monogamous invertebrate despite the potential ability for the female schistosome to store sperm. Thus, we conclude that schistosomes are socially but not genetically monogamous