Two rhizobial strains, BSA136Tand BSA150, related to the genus Mesorhizobium were isolated from root nodules of Lotus tenuis grown in saline-alkaline lowlands soil from Argentina. These strains showed different repetitive element palindromic PCR fingerprinting patterns but shared more than 99% sequence similarity for both 16S rRNA and recA genes. Despite the symbiotic nodC gene sequences of our strains being related to the canonical Lotus biovar species comprising Mesorhizobium loti and Mesorhizobium japonicum, the 16S rRNA phylogenetic marker suggests that their taxonomical identities are closely related to Mesorhizobium helmanticense, Mesorhizobium metallidurans, Mesorhizobium thianshanense, Mesorhizobium gobiense and Mesorhizobium tarimense. Multilocus sequence analysis performed with seven housekeeping genes confirmed that BSA136Tbelongs to a separate clade within the genus Mesorhizobium. The results of comparisons for in silico DNA-DNA hybridization and average nucleotide identity indexes between the genomes of BSA136Tand closest-related Mesorhizobium species were below the threshold for species delineation. Phenotypic features differentiated BSA136Tfrom its closestrelated species. On the basis of our results, BSA136Tand BSA150 can be considered to represent a novel species of the genus Mesorhizobium, for which the name Mesorhizobium sanjuanii sp. nov. is hereby proposed. The type strain of this species is BSA136T(=CECT 9305T=LMG 30060T), for which the draft genome sequence is available.