In experiments to determine the compatibility between isolates of Schistosoma haematobium and Bulinus truncatus from Mali and Senegal, the parasite isolates were shown to differ in their intermediate host specificity. Bulinus truncatus from the Lower and Middle Valleys (Senegal) and the Upper Valley (Mali) of the Senegal River Basin (SRB) were all susceptible to S. haematobium isolated from the urines of children living in Tenegue, Office du Niger, Mali. However, none of the B. truncatus tested was susceptible to a parasite isolated from children living in Mbodiene, a village in the Lower Valley of the SRB, where natural transmission is normally associated with B. globosus. As B. truncatus is widely distributed in the SRB, the possible appearance of a Bulinus-truncatus-borne parasite in the Middle and Lower Valleys should be carefully monitored.