We evaluated the ability of Mesorhizobium strains that were isolated from the soils of the Salado River Basin in the province of Buenos Aires. Two populations of Lotus glaber, FA2 and FA7, were inoculated with three rizobia isolates L10, L15 and L22. The autoctonous strains nodulated and fixed more nitrogen than uninoculated (negative) and USDA3471 inoculated (positive) control plants. The populations of Lotus glaber differed in their ability to grow in the presence of low levels of available nitrogen. We concluded that the soils of the Salado river basin host bacteria with high nitrogen fixing potential that may be used in inoculant production.