Seven ureaplasma strains isolated from the oral cavities of domestic cats (Felis domestica) were characterized and compared with the type strains of the three previously established species of this genus, Ureaplasma urealyticum (humans), Ureaplasma diversum (cattle), and Ureaplasma gallorale (chickens). The feline strains hydrolyzed urea but not arginine or glucose, were membrane bound, lacked cell walls, passed through 0.45-micron membrane filters, required cholesterol for growth, and formed minute (15- to 140-microns) colonies on agar medium. The seven feline strains fell into two distinct groups based on (i) their antigenic properties (determined by using the metabolism and growth inhibition and indirect immunoperoxidase procedures), (ii) their genomic properties (determined by using DNA-DNA hybridization and DNA cleavage pattern procedures), and (iii) their polypeptide profiles (determined by using sodium dodecyl sulfate-polyacrylamide gel electrophoresis analyses). Based on these properties, the two feline groups were unrelated to each other or to the three previously established species, and each group represents a distinct Ureaplasma species. Thus, we propose that ureaplasmas with these phylogenetic and genomic properties be given taxonomic status as Ureaplasma felinum and Ureaplasma cati, with strain FT2-B (= ATCC 49229 = NCTC 11709) and strain F2 (= ATCC 49228 = NCTC 11710) as the type strains, respectively.