The numerical and ecological distribution of free-living, aerobic, nitrogen-fixing bacteria in virgin and cultivated field soils of the Matador IBP Project Site in Saskatchewan, was determined by the plate-count method and the nitrogen-fixing ability of these bacteria, in mixed, enriched populations was estimated by employing the radio-active 13n2 technique. 'In situ' studies were also carried out on the acetylene reducing capacity (an indirect measure of nitrogen-fixation) of these soils in serum bottles as well as in soil columns under constant perculation.