ABSTRACT: The importance of tissue-specificity was studied in Myxobolus spp. infecting the gills of 7 cyprinid species. The 18S rDNA of 10 Myxobolus species was amplified by optimised nested-PCR, resulting in ~1600 bp PCR products. Phylogenetic trees generated by distance matrix and parsimony analyses revealed 4 main groups. Muscle-infecting species all belonged to the same group, while members of 2 gill-infecting groups were clearly distinguishable on the basis of tissue-specificity, and were also recognisable by differences in spore morphology. On the basis of tissue tropism, phylogenetic relationships among the species examined indicate that genetic separation is a more ancient evolutionary feature than host-specificity.