In Africa, the two human schistosome parasites, Schistosoma intercalatum and S. mansoni, can be found sympatrically, which poses the problem of the nature of their reproductive isolation. In this paper, experimental studies of mating behaviour in mixed infections in mice were carried out. Simultaneous infections without choice of mate showed an absence of any physiological barriers preventing encounter and mating of the two species on the same host. Whenever choice was possible, mating did not occur in a random way, but preferentially between partners of the same species (there was a significant prevalence of homospecific pairs). S. mansoni exhibited greater specific mate recognition than S. intercalatum. Moreover, in the absence of S. mansoni female worms, S. mansoni male worms were as competitive as S. intercalatum male worms in pairing with S. intercalatum female worms. It appears from this study that the choice of mate is a reproductive isolating factor between the two species.