Fly load of Onchocerca volvulus microfilariae and their early development in Simulium ochraceum were examined at intervals of 10, 30 and 60 min after ingestion and then hourly up to 72 hours. Many microfilariae ingested were damaged by the cibarial armature of the vector, limiting the fly load, but some of these damaged microfilariae were able to penetrate the gut before they died. Thus, the mean number of the living parasites per fly in the head and thorax decreased gradually with elapse of time after ingestion. The parasites ingested by S. ochraceum did not develop beyond the microfilarial stages during the first 24 hours; thereafter only thoracic forms (lst-stage larvae) were found to be alive. The high mortality of the infected fly might be caused by the high intakes of microfilariae and also by normal and/or partly wounded microfilariae invading various organs. The numbers of microfilaria ingested from different biting sites were usually high in comparison with the densities observed in each site; this indicates that the microfilariae might be attracted by the biting females.