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We studied the rhizosphere populations of soybean ( Glycine max . L.) in two soils throughout the growing season and evaluated the activities of selected organisms on nodule occupancy of serogroup 123 (USDA 123), the dominant Bradyrhizobium japonicum found in mid-U.S.A., soils. Non-specific bacteria and actinomycetes. Gram-negative bacteria, enterobacteria and pseudomonads were enumerated approx. every 3 wk on six occasions during the growing season. Early in the season, 31% of the rhizosphere bacteria were inhibitory to USDA 123 as estimated by an agar-overlay technique, but only 8% were stimulatory. Nine of the stimulatory organisms were tested in the greenhouse in sterile soil for their ability to alter occupancy of soybean nodules by strain USDA 123 when challenged by USDA 110. Two of these rhizosphere organisms significantly enhanced nodulation of USDA 123, but further tests indicated that the results were not reproducible in non-sterile soil. In general, rhizosphere populations in the field increased throughout the season but varied with climatic variables. Populations were positively correlated with precipitation and temperature and negatively correlated with evaporative index.