The genetic and phylogenetic relationships for strains of Mesorhizobium tianshanense and its relatives were compared by an analysis of the results of sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDSPAGE) of whole-cell proteins, DNA-DNA hybridization, and full 16s rRNA gene sequencing. The strains of M. tianshanense formed a cluster which was distinct from those of other rhizobium species in the clustering analysis of SDS-PAGE. DNA-DNA relatedness between A-1BS (type strain of M. tianshanense) and the type or eference strains for Mesorhizobium loti, M. huakuii, M. ciceri, M. mediterraneum, and cluster U, an unnamed hizobial group, ranged from 4.4 to 43.8%. The phylogenetic analysis based on the 16s rRNA gene sequences howed that M. tianshunense was closely related to the Mesorhizobium phylogenetic branch and could be istinguished from the other four species in this branch. These results further confirmed that these bacteria onstitute a distinct rhizobial species.