Human onchocerciasis is hyper-endemic in south-western Cameroon and the island of Bioko. The vector on the island is the endemic 'Bioko' form of Simulium yahense. On the mainland, S. damnosum s.s., S. squamosum and S. mengense are recorded around Mount Cameroon but their contribution to transmission in the area is unclear. As elimination of onchocerciasis, through vector eradication, is a possibility on the island, it is important to be able to identify vector flies which might migrate to Bioko from the mainland. The morphologies of S. damnosum s.s. and S. squamosum are already well known. New cytotaxonomic and morphotaxonomic descriptions of S. mengense, a species which is less well known, are presented. Simulium mengense can be distinguished from the other cytospecies in the area by the presence of tufts of hair-like setae on the larval thorax, the presence of hairs on the subcostal vein of the adult female, and by the scutal pattern of the adult male. Although Vajime and Dunbar described seven fixed inversions in S. mengense, in 1977, nine were observed in the present study. Of the nine, three were the same as Vajime and Dunbar's but the other six were either missed or misinterpreted by them.