Natural and agricultural systems are nitrogen deficient and therefore often depend for productivity upon atmospheric nitrogen fixed in nodules formed on legumes by root nodule bacteria. The commonly accepted figure for global nitrogen fixation in agriculture is 25-90 million metric tons, worth in excess of U.S. $10 billion. Rhizobia associated with annual pasture legumes contribute a substantial proportion of this fixed nitrogen, and the amount of nitrogen fixed annually by the Sinorhizobium-Medicago symbioses is estimated to be worth $250 million. Two sinorhizobial species are the focus of intensive research: S. meliloti and S. medicae. The latter strain can be distinguished from the former by its unique capacity to fix nitrogen in association with annual and perennial Medicago hosts (including alfalfa) of worldwide agronomic value. DNA hybridization experiments reveal no more than 60% homology between the two species, explaining many physiological and genetic differences. Thus, the available S. meliloti Sm1021 sequence, by itself, is of limited value for studies concerning S. medicae.