Summary Several hundred specimens each of Simulium exiguum, S. veracruzanum, and S. haematopotum were allowed to feed on volunteers heavily infected with microfilariae of Onchocerca volvulus, and were subsequently dissected after various intervals of time. All three species proved capable of supporting the development of the parasite and must therefore be considered potentially capable of transmitting human onchocerciasis in those parts of the endemic zone where they replace or supplement S. ochraceum, S metallicum, and S. callidum, the three species commonly accepted as intermediate hosts.