Different interests between mating partners regarding the fate of their gametes can lead to sexual conflicts in many species. Although these conflicts can sometimes be dealt with pre-copulatorily (e.g. by choosing with which partners to mate), they often extend beyond copulation. Post-copulatory sexual conflicts are expected to be particularly strong in simultaneous hermaphrodites because an individual may have to accept sperm in order to obtain an opportunity to donate sperm, reducing the effectiveness of pre-copulatory conflict resolution. The present study investigates the post-copulatory interactions between male and female sexual traits of a highly promiscuous simultaneous hermaphrodite, the free-living flatworm Macrostomum lignano. Using light and electron microscopy, we show the different levels of complexity of the sperm and the genitalia, and derive hypotheses about how the different traits may represent evolutionary responses to such sexual conflicts. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99, 370–383.