Fourteen serotypes (serovars) are currently recognized in the species Ureaplasma urealyticum. DNA-DNA hybridization tests, cleavage of genomic DNA by restriction endonucleases, and polyacrylamide gel electrophoresis of cell proteins indicate that the U. urealyticum serotypes fall into two genotypically distinct, but related, clusters. Southern hybridization tests of mycoplasmal ribosomal RNA gene probes with DNAs of the various serotypes digested by restriction endonucleases showed that the U. urealyticum chromosome carries two sets (operons) of ribosomal RNA genes. However, the hybridization patterns produced by the serotypes showed differences which were most pronounced among serotypes belonging to the two different clusters.