Eleven strains that formed symbiotic root nodules on Alhagi sparsifolia, designated previously as genospecies II, were identified as a new lineage of Mesorhizobium (Alphaproteobacteria) that could be differentiated from all previously recognized species of the genus Mesorhizobium by using 16S rRNA gene sequences (,97.8 % similarity), DNA–DNA hybridization (,45 %), dnaJ, dnaK, recA, glnA, nifH, nodA and nodC gene sequences, fatty acid profiles (C 18 : 1 v7c, 35 %;11-methyl C 18 : 1 v7c, 30 %) and numerical taxonomy. These strains are therefore considered to represent a novel species, for which the name Mesorhizobium alhagi sp. nov. is proposed, with isolate CCNWXJ12-2 T (5ACCC 15461 T 5HAMBI 3019 T) as the type strain.