Atmospheric nitrogen fixation by perennial soybean (Glycine wightii Verdc.) and siratro (Phaseolus atropurpureus moç. & sesse) in symbiosis with indigenous rhizobia of two soils of the State of Sao PauloExperiments were carried out in Leonard's jars, with perennial soybean and siratro, to evaluate the nitrogen fixation ability of these legumes when in symbiosis with the population of Rhizobium sp. indigenous to two great groups of soils of São Paulo State, Brazil. The results showed that the rhizobia occuring in a "Latosolic B Terra-Roxa" and in a "Red Yellow Latosol sandy phase", under "cerrado" vegetation, are fairly ineffective, when compared with the Rhizobium inoculated control. It has also been shown, particularly for siratro, that previous cropping with perennial soybean on the "Latosolic B Terra-Roxa" had a favourable effect on the natural population of Rhizobium sp.