Phymatolithon acervatum is an encrusting, non-geniculate coralline alga from South Africa. While morpho-anatomical details of the species are well documented, no genetic analyses have been performed on P. acervatum. Here, we analyzed rbcL and psbA gene sequences, and they showed that two species were passing under the name Phymatolithon acervatum in South Africa. A partial rbcL sequence from the lectotype of Lithothamnion acervatum, basionym of P. acervatum, showed to which species the specific epithet applies, and furthermore that both species belong in Phymatolithopsis. We transfer Phymatolithon acervatum to Phymatolithopsis, as Phymatolithopsis acervata comb. nov., and propose Phymatolithopsis roseola sp. nov. These species overlap in morpho-anatomy, habitat and distribution and can only be distinguished based on DNA sequences. Previous misinterpretations of conceptacle development are corrected for species of Phymatolithopsis. The main character used to define the family Mesophyllumaceae is shown to have arisen several times in independent lineages in the order Hapalidiales. At present, only one family, Hapalidiaceae, thus merits recognition in the Hapalidiales.