Fish-parasitic myxosporeans comprise an extraordinarily large number of species. The genus Myxobolus alone, which is the best studied genus of Myxosporea, was reported to contain 444 species (Landsberg and Lorn 1991). Accurate determination of the number of species is rendered difficult by the fact that taxonomic classification is based on the morphology of spores showingonly slight structural differences, and that the development and the species, organ and tissue specificity of the parasites are little studied.