The commonly reported freshwater snail Ceratophallus natalensis (Krauss) (Planorbidae) of eastern and southern Africa is shown here to be a composite taxon comprising at least three species. The species of Ceratophallus are found only in Africa and some Indian Ocean islands; they are distinguished by the structure of the penis, which is chitinized internally and may bear microspination. Ceratophallus natalensis (Krauss) has been often recorded as common in pools and marshes. According to observations on the genital organs reported here, this taxon comprises three species, C. natalensis, C. gibbonsi (Nelson) comb. nov. and C. yesimit sp. nov. No clear-cut conchological differences among them were evident. Anatomical observations were made on snails from 398 collectors' samples, providing many new locality records for this genus (details in Appendix). Ceratophallus natalensis and C. gibbonsi are widespread from Ethiopia to South Africa, whereas C. yesimit is known only from the Ethiopian Rift Valley. Though natalensis and gibbonsi are partly sympatric, the latter is associated with fully tropical climatic areas and comparatively permanent waterbodies, whereas natalensis is commonest in cooler areas and is abundant in seasonal rain pools. This study showed the penis of Ceratophallus to be stiffened by internal chitinous bands that apparently are unique in the Planorbidae and perhaps for all pulmonate molluscs. It is suggested that the microspinous penis of C. natalensis may be adapted to the function of sperm displacement. The chromosome number 2n=36 was observed for C. natalensis. Comparative data are given for Afrogyrus coretus (de Blainville) (Planorbidae), which in part of its range is sympatric with Ceratophallus and has a similar shell, but differs in having an external penial stylet. Although both groups are found only in the Afrotropical Region, they apparently differ in biogeographical history and phylogeny. In a cladogram where the first apomorphy is external chitinization of the penis, Afrogyrus is allied to Gyraulus, whereas Ceratophallus is more closely related to Planorbis.