The phenotypic and genotypic properties of Ureaplasma urealyticum (family Mycoplasmataceae, order Mycoplasmatales, class Mollicutes) are reviewed here. The 14 recognized serovar standard strains found in humans exhibit no serological cross-reactivity with ureaplasmas from other hosts and uniquely express human immuoglobulin A1 protease activity. However, they exhibit many characteristics which place them in two distinct clusters known as the parvo biovar (or biovar 1 or B) and the T960T biovar (or biovar 2 or A). Established phenotypic markers of the biovars include clustering of antigenic types, polypeptide patterns of whole-cell preparations, differential inhibition by manganese, and polymorphism among their ureases, pyrophosphatases and diaphorases. Established genotypic markers of the biovars are DNA-DNA hybridization of 60% between biovars, and distinctive RFLP patterns and genome sizes. Divergent nucleotide sequences of several highly conserved genes attest to the phylogenetic distinctiveness of the two biovars. PCRs founded upon the sequences for 16S rRNA, the 16S-23S rRNA intergenic regions, the genus-defining urease, the serovar-defining, multiple-banded antigen genes or randomly amplified polymorphic DNA tests differentiate the biovars unambiguously. With the availability of rapid, reliable and economical tests for biovar determination, it is now appropriate to propose that the taxonomic status of U. urealyticum be emended. Serovar standard strains exhibiting traits of biovar parvo (serovars 1, 3, 6 and 14) will be designated as a separate species, Ureaplasma parvum sp. nov., as befits its smaller genome size. The serovar 3 standard (strain 27T) will be the type strain of U. parvum and is represented by ATCC 27815T and NCTC 11736T. Serovar standard strains exhibiting traits of biovar T960T (2, 4, 5, 7, 8T, 9, 10, 11, 12 and 13) will retain the U. urealyticum designation and type strain, the serovar 8 standard (strain T960T), represented by ATCC 27618T and NCTC 10177T.