We performed a numerical analysis of 148 phenotypic characteristics of 20 strains of root nodule bacteria isolated from an arid saline desert soil in the Xinjiang region of northwestern People's Republic of China and compared these organisms with 28 Rhizobium and Bradyrhizobium strains obtained from different regions of the People's Republic of China and from other countries, including nine type strains of different species. All of the strains examined clustered into two groups at a similarity level of more than 63%. Group I included all of the previously described Rhizobium species and was divided into eight subgroups, which corresponded to previously described Rhizobium species, at a similarity level of more than 82%. Group I1 was divided into the following three subgroups at a similarity level of more than 80% Bradyrhizubium japonicum, a cluster containing 17 moderately and slowly growing strains isolated in the Xinjiang region, and a small subgroup containing three fast-growing strains. The generation times of the moderately and slowly growing strains were 5 to 15 h, and these organisms produced acid in medium containing mannitol. The DNA G+C contents of the members of this group ranged from 59 to 63 mol%. DNA-DNA hybridization experiments revealed that the levels of DNA homology among all of the moderately and slowly growing strains obtained from Xinjiang were more than 70% and that the levels of DNA homology between representative strains of this group and the type strains of all previously described species of root-and stem-nodulating bacteria were low. All of our experimental data indicated that the moderately and slowly growing rhizobia isolated from Xinjiang are members of a new species. Partial 16s rRNA gene sequencing of the type strain. A-1BS (= CCBAU3306), and a comparison of the resulting sequence with the sequences of previously described species revealed that strain A-1BS is closely related to Rhizobium loti, Rhizobium huakuii, and Rhizobium galegae, but not to B. japonicum and Azorhizobium caulinodans. On the basis of our results, we propose that the strains which we studied are members of a new species, Rhizobium tianshanense. The type strain, A-1BS (= CCBAU3306), has been deposited in the Culture Collection of Beijing Agricultural University, Beijing, People's Republic of China.At the present time, all bacteria that induce nodules on the roots of leguminous plants are assigned to three genera (8,14). The genus Rhizobium comprises a group of fast growers and includes eight species, Rhizobium leguminosarum, Rhizobium meliloti, Rhizobium loti (14), Rhizobium fredii (20), Rhizobium huakuii (2), Rhizobium galegae (15), Rhizobium tropici (16), and Rhizobium etli (21). Each of these species is associated with a group of host plants and is distinguished from the other species mainly on the basis of DNA relatedness values, 16s rRNA homology values, and some phenotypic characteristics. The genus Bradyrhizobium comprises a group of slow growers and contains one species, Bradyrhizobium japonicum, which i...