Two of the four currently recognised species of Phymatolithon in South Africa are P. repandum (type locality: Half Moon Bay, Port Philip Bay, Victoria, Australia) and P. ferox (type locality: Natal, KwaZulu-Natal, South Africa). Partial rbcL sequences of the lectotype of P. repandum and the holotype of P. ferox demonstrate that the former is not present in South Africa and does not belong in Phymatolithon, whereas the latter belongs in Phymatolithon, but that the name has been misapplied in South Africa. The correct name for the species, currently called P. repandum in South Africa, is P. ferox. Partial rbcL and psbA sequences of two of the current heterotypic synonyms of P. ferox, Lithothamnion prolixum and Lithothamnion prolixum f. plicatum, show that they are conspecific and do not belong in Phymatolithon. Lithothamnion prolixum is the basionym for the species currently, but incorrectly, called P. ferox in South Africa. Phymatolithon repandum and L. prolixum belong in an as yet undescribed genus. All heterotypic synonyms of currently recognised names of non-geniculate corallines that were placed in synonymy based on morpho-anatomy should not be accepted until confirmed by DNA sequencing.