The nucleotide sequences of the spacer regions between the 16S and 23S rRNA genes of six Ureaplasma species were determined following amplification by polymerase chain reaction. The hypothetical secondary structure of the spacer regions was divided into four domains. Domains I and III were highly conserved among different Ureaplasma species but domains II and IV were variable. The intergenic spacer regions of ureaplasmas lacked spacer tRNA genes. Phylogenetic relationships among the Ureaplasma species indicated that the evolutionary history of the Ureaplasma species follows that of the host animal species, suggesting coevolution between ureaplasmas and animals.