Schistosoma mansoni and S. intercalatum, two schistosomes from different evolutionary lineages, are parasitic in humans and therefore able to co-infect the same host where they occur sympatrically in Africa. Previous studies of mating interactions between these species in mice, using the Lower Guinea strain of S. intercalatum, have demonstrated the competitive dominance of S. mansoni over S. intercalatum in terms of pairing ability, which is potentially an important mechanism restricting the distribution of S. intercalatum in Africa. The study presented here examines the mating interactions in mice between S. mansoni and the Zaire (Democratic Republic of Congo) strain of S. intercalatum, which differs from the Lower Guinea strain in many biological characteristics. Analysis of the data showed a preponderance of intraspecific pairs over interspecific, demonstrating a specific mate preference system for both species. Mating competition between these species and the ability of males of both species to effect a change of mate by pulling paired females away from their partners was indicated. Comparisons are made between the competitive mating abilities of both strains of S. intercalatum relative to those of S. mansoni, with the data suggesting that S. mansoni is competitively dominant to S. intercalatum (Zaire) in sequential infections but to a lesser extent than for S. intercalatum (Lower Guinea). Additional factors which may contribute to the confinement of S. intercalatum (Zaire) to the Democratic Republic of Congo are discussed.