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The effects of temperature on growth in broth and soil and on competition for nodule formation betweenRhizobium japonicum serotypes USDA 76 and 94 compared to 6 and 110 were studied. Increasing root temperatures of Lee soybean from 20 to 35°C increased the competitiveness of 76 and 94 relative to 6 and 110 for all inoculum ratios such that at 30 and 35°C symptoms ofRhizobium-induced chlorosis appeared. Tolerance to elevated temperatures was exhibited by 76 and 110, but not 94 and 6 in broth and soil which suggested that increased competitiveness of 76 and 94 at high soil temperatures was not dependent upon growth at elevated temperatures. Nodulation and vegetative growth of Lee soybeans were at a minimum at 20°C and optimum at 30°C. Differences in competitiveness of 6 to previous studies indicated the need to standardize temperatures of assays. Differences in growth responses of 76 and 94 to temperature from a previous study suggested a confounding effect on different carbon sources in growth media.