A survey was carried out on natural parasites of the onchocerciasis vectors Simulium sanctipauli s.1. and S. yahense in the rain forest zone of Liberia. Apart from filariae the most prevalent parasites inhabiting adult females were fungi (Phycomycetes) of unknown taxonomic classification which infect the ovaries of their hosts. Development takes place in the calyx of the ovaries and ends in the formation of spores. Such ovarian mycoses were particularly common in the rainy season. It is assumed that they considerably reduce the longevity of the flies and therefore also transmission of onchocerciasis during the rainy season. Further parasites seen were ciliates assigned to the Tetrahymena group, sporozoan-like organisms, inhabiting S. yahense, and nematodes.