East African and south African churchills (Petrocephalus, Mormyridae) were synonymised in 1959 to become members of a single species of subcontinental, southern African distribution, Petrocephalus catostoma (Gunther, 1866). By comparison with the type material for P. catostoma from the Ruvuma River and P. stuhlmanni from the Ruvu River, both of East African origin, we confirm the South African form of churchill to represent a new species, P. wesselsi, ranging from the northern Limpopo and Incomati systems south to the Pongola River (Natal) as its southern limit. We also compared churchills from the Sabie River (25 degrees S, South Africa, Incomati system) with churchills from the Upper Zambezi River (17 degrees S, Namibia), using electric organ discharges (EODs) and morphology. The duration of an EOD pulse of the South African form (N = 39; 943.2 +/-S.E. 18.82 mus) is, on average, more than twice that of the Upper Zambezi form (N = 37; 436.6 +/- 15.1 mus), and the amplitude of the second head-positive phase (P2 phase relative to P1 = 1) significantly weaker (0.133 +/- 0.0005 vs. 0.472 +/- 0.002 for Upper Zambezi males, 0.363 +/- 0.03 for Upper Zambezi females). In contrast to the Upper Zambezi form, the EOD of the South African form exhibits no difference between the sexes. Fish from the two origins differ significantly in 11 out of 14 anatomical characters studied, confirming molecular genetic differentiation on the species level.