Ten-day-old adults of Echinostoma caproni and E. paraensei, some of which had been exposed to 3-H-tyrosine to label sperm, were transplanted to mice in various combinations. Mating behaviour was followed on autoradiograms of worms recovered after 5 days by detecting the transfer of labelled sperm from labelled worms to themselves or to unlabelled worms. When single, labelled E. caproni adults were transplanted with unlabelled E. paraensei, they self-inseminated but did not show evidence of interspecies mating. No interspecies mating but self-insemination was observed when single, labelled E. paraensei and unlabelled E. caproni were transplanted together. When the labelled species had a choice of unlabelled adults of its own species or the opposite species, it mated in similar fashion to that seen in single species infections. The labelled species acted as if the opposite species was not present and showed an unrestricted mating pattern where it would both self- and cross-inseminate. Even though after transplant the E. paraensei adults were found in the duodenum and the E. caproni adults were found in the ileum, approximately 25% of the transplanted worms of both species were found within 1 cm of each other. Thus habitat separation was not a major cause of the lack of interspecies mating. Lack of reproductive recognition is another reason to separate E. caproni and E. paraensei into distinct species.