Five rhizobial strains representing the largest group in the genus Mesorhizobium associated with Caragana spp. in China were characterized taxonomically. Phylogenetic analysis based on 16S rRNA gene sequences indicated that these microsymbionts belonged to the genus Mesorhizobium, with Mesorhizobium tianshanense USDA 3592(T), Mesorhizobium temperatum SDW018(T) and Mesorhizobium mediterraneum UPM-Ca36(T) as the closest neighbours (>/=99.5 % 16S rRNA gene sequence similarity). Genotypic fingerprinting by whole-cell protein electrophoresis, DNA-DNA hybridization, comparative housekeeping sequence analysis of the atpD, glnII and recA genes, fatty acid profiles and a series of phenotypic and physiological tests allowed the novel group to be differentiated from all previously recognized species of the genus Mesorhizobium. This group therefore represents a novel species, for which the name Mesorhizobium caraganae sp. nov. is proposed with the type strain CCBAU 11299(T) (=LMG 24397(T)=HAMBI 2990(T)). Cross-inoculation tests showed that strain CCBAU 11299(T) could form effective nodules on Caragana microphylla, Caragana intermedia, Glycyrrhiza uralensis, Astragalus adsurgens and Phaseolus vulgaris.