ABSTRACT.    Trichomitus batrachorum (Parabasala, Trichomitidae) was originally described from intestine of frogs. However, morphologically similar flagellates considered to be an independent species, occur also in reptiles and horse leeches, were once found even in human intestine. Due to the huge morphological and host diversity, it is unclear whether all these forms represent a single species with unusually broad host range, or whether T. batrachorum is in fact a complex of several host-specific species. To this end, we have isolated and cultivated 23 Trichomitus isolates from 19 species of lizards, turtles, tortoises, frogs, leeches, and lemurs, sequenced their SSU rDNA and the ITS1-5.8S rRNA-ITS2 region and performed a phylogenetic analysis. Our isolates form two well-supported clades. The first one comprises all lizard isolates, one isolate from lemur and one isolate from frogs. The second clade is formed by the remaining isolates from frogs, as well as by two isolates from horse leeches. The genetic distance separating the two clades and even some distances within the clades exceed considerably usual interspecific distances among trichomonads. We therefore conclude that organisms assigned to Trichomitus batrachorum may represent two complexes of species with limited host specificity and that at least one lineage naturally infects both frogs and horse leeches.