ABSTRACT.    The genus Hypotrichomonas Lee, 1960 (Parabasala, Trichomonadida) is unique among parabasalids by possessing a rudimentary costa and a fully developed undulating membrane. Its closest relative, the genus Trichomitus, possesses a fully developed costa. Although the evolution of costa is not fully elucidated, it is widely accepted that it is secondarily reduced in case of Hypotrichomonas. Four Hypotrichomonas species from intestine of lizards and one species from birds have been described, though most of the descriptions are probably not valid and only H. acosta, the type species from tropical lizards, has been observed repeatedly. We have examined about 90 previously obtained trichomonad isolates from various hosts and among them we have discovered fourteen Hypotrichomonas isolates from marsupials, rodents, primates, pigs, tortoises, lizards, frogs and cockroaches. We have sequenced their SSU rDNA and have stained them with protargol. Both morphological and phylogenetic analyses showed that our isolates represent eight species: H. acosta and probably H. osmaniae from lizards and frogs, and six new species from the other host taxa. Moreover, several SSU rDNA sequences from GenBank obtained from intestine of a passalid beetle formed two separate lineages within the genus Hypotrichomonas. Our results show that the genus Hypotrichomonas is much more diverse than previously thought and that it contains at least ten species which differ in morphology, phylogenetic position and host range. Interestingly, undulating membrane of the species from pigs is almost completely reduced and the cells are superficially similar to cells of the genera Monocercomonas and Honigbergiella. Further investigation of ultrastructure of the new Hypotrichomonas species could bring new information on the evolution of costa and undulating membrane.