Predictions of mating system and sexual conflict theory are applied to mating systems of the best studied among the simultaneously hermaphroditic, sequentially hermaphroditic and gonochoric species of the dorvilleid genus Ophryotrocha and of two species of dinophilids. In the hermaphroditic Ophryotrocha species, the mating system is characterized by pair mating, absence of sperm competition and exchange of sexual roles between partners (egg trading). Features that stabilize the pair bond are analyzed (i.e. breeding sex-ratio, biparental care, mechanisms against cheating). Male-male competition and female preference for small males differentiate the mating system of a sequentially hermaphroditic species from that of a gonochoric species, where females prefer large males as mates. In the gonochoric species of Dinophilus different population structures (either panmictic or with sib-mating) select either for absence of sexual dimorphism and a 1:1 sex ratio or for strong sexual dimorphism and female biased sex ratio.