Four rhizobial strains, designated CCBAU 85046(T), CCBAU 85051, CCBAU 85048 and CCBAU 85049, isolated from root nodules of Oxytropis glabra grown in Tibet, China, were previously defined, using amplified 16S rRNA gene restriction analysis, as a novel group within the genus Rhizobium. To clarify their taxonomic position, these strains were further analysed and compared with reference strains of related bacteria using a polyphasic approach. The 16S rRNA gene analysis showed that the four isolates formed a distinct phylogenetic lineage in the genus Rhizobium. The isolates showed highest sequence similarity (97.8 %) to Rhizobium indigoferae CCBAU 71042(T). Phenotypic and physiological tests, DNA-DNA hybridization, phylogenetic analyses of housekeeping genes recA, atpD and glnII and fatty acid profiles also indicated that these four strains constitute a novel group distinct from recognized species of the genus Rhizobium. Based on this evidence, strains CCBAU 85046(T), CCBAU 85051, CCBAU 85048 and CCBAU 85049 represent a novel species in the genus Rhizobium, for which the name Rhizobium tubonense sp. nov. is proposed. The type strain is CCBAU 85046(T) (=LMG 25225(T) =HAMBI 3066(T)) and its DNA G+C content is 59.52 % (T(m)). Strain CCBAU 85046(T) could form effective nodules on plant species Vigna unguiculata and Medicago sativa but not on its host of origin Oxytropis glabra.