Prevalence of human onchocerciasis in Tanzania is described. The prevalence is high in Bwakira (63.6 per cent), Mahenge (58.6 per cent) and Ruvuma (31.9 per cent); and low in Amani (22.4 per cent) foci when compared to that reported in 1970. The increase and decrease on the prevalence indicate that the disease transmission has been increasing and decreasing in the respective foci. It is speculated that bush clearing and deforestation in general, has positive effects on the transmission of onchocerciasis where Simulium damnosum complex is a vector and negative effects where Simulium neavei complex exists.