The available data on the serologic and genomic relationships among the established and unspeciated Ureaplasma species and serovars isolated from various hosts can be summarized as follows. Ureaplasma urealyticum (human) is composed of 14 serovars separated into two genomic clusters. Ureaplasma diversum (bovine) is antigenically complex, has three serologic clusters and requires the three representative antisera to identify all U. diversum strains. The nonhuman primate strains form four serologic groups, and each serogroup is composed of strains isolated from primates belonging to one of four distinct zoologic primate families. The ovine-caprine strains have two serologic clusters. Canine strains form four serologic clusters but serovars 1 and 2 are closely related by DNA homology. Avian strains belong to one serogroup with two genomic clusters. The DNA homology data indicate that phenotypic information alone, including antigenic serotypic data, is not always adequate for species designation among the ureaplasmas and that comparative analyses of the genome provide invaluable data for establishing new species. Although there are only two established species, the published data support the contention that the nonhuman primate, ovine-caprine, canine, feline and avian ureaplasmas are genomically and phenotypically distinct from each other based on the serologic, polyacrylamide gel electrophoresis protein and DNA cleavage patterns and DNA homology studies and that these ureaplasmas from various hosts may represent new species or subspecies within the genus.