Ribonuclease P (RNase P) RNA genes of LL-2,6-diaminopimelic acid (LL-A2pm)-containing actinomycetes, except Streptomyces species, were sequenced after PCR-amplification and cloning. By using the sequence data, the relationships between species within genera and the relationships between taxa above genus level were investigated and the usefulness of the RNase P RNA gene as another phylogenetic marker was evaluated. RNase P RNA gene sequences of all strains used in this study contained relatively conserved regions along with highly variable regions. The mean RNase P RNA gene similarity value was approximately 82 +/- 18% and the mean RNase P RNA gene similarity value when gaps were included was approximately 76 +/- 24%. The nucleotide similarities between the RNase P RNA genes of different strains were mostly fewer than the 16S rDNA similarities. The RNase P RNA gene was more useful than 16S rDNA for clearly differentiating the relationships between species belonging to a genus and the relationships between some genera. However, nucleotide sequences of RNase P RNA genes were not necessarily appropriate for comparisons at all taxonomic levels (such as those between species, between genera and between families).