Fungal species identities are often based on morphological features, but current molecular phylogenetic and other approaches almost always lead to the discovery of multiple species in single morpho-species. According to the morphological species concept, the ant-parasitic fungusRickia wasmannii(Ascomycota, Laboulbeniales) is a single species with pan-European distribution and a wide host range. Since its description, it has been reported from ten species ofMyrmica(Hymenoptera, Formicidae), of which two belong to therubra-group and the other eight to the phylogenetically distinctscabrinodis-group. We found evidence forR. wasmanniibeing a single phylogenetic species using sequence data from two loci. Apparently, the original morphological description (dating back to 1899) represents a single phylogenetic species. Furthermore, the biology and host-parasite interactions ofR. wasmanniiare not likely to be affected by genetic divergence among different populations of the fungus, implying comparability among studies conducted on members of different ant populations. We found no differences in total thallus number on workers betweenMyrmicaspecies, but we did observe differences in the pattern of thallus distribution over the body. The locus of infection is the frontal side of the head inMyrmica rubraandM. sabuletiwhereas inM. scabrinodisthe locus of infection differs between worker ants from Hungary (gaster tergites) and the Netherlands (frontal head). Possible explanations for these observations are differences among host species and among populations of the same species in (i) how ant workers come into contact with the fungus, (ii) grooming efficacy, and (iii) cuticle surface characteristics.