Nine strains isolated from symbiotic root nodules on Alhagi sparsifolia were previously designated as representing genospecies I. Phylogenetic analyses indicated that genospecies I was related closely to Mesorhizobium alhagi (genospecies II), and clearly formed a new lineage within the genus Mesorhizobium. In this study, we differentiated genospecies I from recognized species of the genus Mesorhizobium based on phylogenetic analyses of additional core genes (recA, glnA), levels of DNA-DNA relatedness (<43.3 %), fatty acid profile (58 % C₁₈:₁ω7c, 19 % 11-methyl C₁₈:₁ω7c), and biochemical and physiological characteristics. The nine strains are therefore considered to represent a novel species of the genus Mesorhizobium, for which the name Mesorhizobium camelthorni sp. nov. is proposed. The type strain is CCNWXJ 40-4(T) (=HAMBI 3020(T) =ACCC 14549(T)).