rpoB sequences encoding the beta-subunit of RNA polymerase were determined in 26 Mycoplasma species for phylogenetic study. The portion of rpoB DNA used in this study showed a high degree of variation in terms of size and sequence among species. The rpoB phylogenies inferred from amino acid and nucleotide sequences were used to divide the mycoplasmas into two groups, a 'pneumoniae group' and a 'hominis group', which was consistent with the result from 16S rDNA sequence analysis. However, phylogenetic relationships within these groups differed in the two gene trees, which were supported by the incongruence length difference (ILD) test. This indicates that multiple gene sequences must be applied to infer accurate phylogenetic relationships among the mycoplasmas. The rpoB sequence, and especially the deduced amino acid sequence, offers a good alternative marker.