The low cure rates obtained with praziquantel in a Senegalese focus of schistosomiasis can best be interpreted on the basis of epidemiological factors, and are unlikely to be connected with any drug resistance in the parasite. Schistosome isolates obtained in Egypt from uncured patients present evidence of lower susceptibility to the drug, albeit to a rather limited extent. Similarly, laboratory schistosomes subjected to repeated passages under drug pressure are partly insensitive to the drug. Oxamniquine is at present the only available alternative to praziquantel. Research and development of new antischistosomal drugs is urgently needed.