A total of 33 strains of fast-growing soybean rhizobia isolated from soil and soybean nodules collected in China and 25 strains beioning to the genera Rhizobium Radythizobium and Agrobacterium were compared by numerical taxonomic techniques using 240 different characters Ou resultsindicatied that all of the strains of fast-growing soybean rhizobia which we examined are closely related guanine-plus-eytosine content, 59.9 to 63.8 mol% and are separated from Rhizobium and Bradyshizobium at the generic level Based on numerical taxonomy deoxyribonucleic acid (DNA) base ratio determinations DNA-DNA hybridization data serologica analysis data the composition of extracellultar gum bacteriophage typing data and soluble rpotein patterns we propose that the fast growing soybean rhizobia represent members of a new genuw rather that a species of Rhizobium Rhizobium fredio; we propose Sinorhizobium gen nov as an appropriate generic name The type species of the new genus is Sinorhizobium fredii comb nov (basonym Rhizobium fredii Scholla and Elkan 1984) and the type strain is strain ATCC 35423 (=USDA 205) For the other species we propose the name Sinorhizobium xinjaingenis sp. Nov.; the type strain of this pecies is strain CCBAU 110, which has been deposited in the Beijing Agricultural Univesity Culture Collection Beijing People's Republic of China.