This research investigated the symbiotic effectivity and competitiveness of three strains of Chinese fast-growers (Rhizobium fredii) in the greenhouse and in the field, and evaluated the competitive ability of siderophore over-producing mutants of Chinese fast-growers (developed through transposon-mediated mutation) in an alkaline soil of Iowa. In the greenhouse study on effectiveness, three Chinese fast-growers were tested along with two USDA slow-growers (Bradyrhizobium japonicum) on four soybean cultivars. There was a significant strain-by-cultivar interaction, and two of the fast-growers were as effective as USDA 123 in N fixation on most of the cultivars, but less effective than USDA 110 on all cultivars tested. In the competition studies, the competitiveness of the fast-growers was evaluated in the greenhouse against native bradyrhizobia in six different soils. Nodule occupancy by fast-growers ranged from 1.5 to 38.8%, and there were significant interactions among strains, soils, and cultivars. For field testing, two separate sites each year were selected in the summers of 1987 and 1988, and each of the three fast-growers was introduced into soil at approximately 10[superscript]6 viable cells cm[superscript]-1~ row. Nodule occupancy over the four site-years on the cultivars Corsoy 79 and Williams 82 ranged from 3.8 to 13.3%. Both cultivars responded to N fertilizer with higher yields than the noninoculated controls, which indicated that the native slow-growers were not fixing all the N needed for maximum growth of the plants. The fast-growers persisted reasonably well as saprophytes over a 3-yr period based on nodule occupancy in the greenhouse of field-collected soil. In the siderophore study, only USDA 135 of the three slow-growers tested produced siderophore in liquid culture, which could explain their dominance in alkaline soils where availability of iron is presumed low. In an alkaline soil, two siderophore over-producing mutants, which had retained their symbiotic characteristics and tested positive on Southern hybridization with a Tn5 probe, occupied only one-sixth as many nodules as the wild-type. The symbiotic effectivity of the mutants, however, was not affected by mutation induced by Tn5 insertion. Further characterization of the mutants is necessary to understand why the mutants occupied fewer nodules.