The occurrence of free-living nitrogen fixers, the potential for nitrogen fixation, and the correlation between the nitrogen-fixing capacities of the soils and bacterial counts were studied using representative Quebec soils.Clostridiumoccurred more frequently than didAzotobacter. Studies with N15showed that nitrogen fixation was more frequent under anaerobic than under aerobic conditions in all the soil types studied in their unamended state. The addition of glucose stimulated nitrogen fixation. During anaerobic incubation, nitrogen fixation was found to be correlated significantly with the increase in numbers of both total aerobes and Clostridia. The results suggested that facultatively anaerobic nitrogen fixers, and aerobic nitrogen fixers other thanAzotobacter, were present.