This research investigated the characterization of native Bradyrhizobium japonicum serocluster 123 isolates from Iowa soils. Twenty-one strains of virulent bacteriophages were isolated against 37 B. japonicum isolates and strains USDA 122, 123, 127, and 129. Each isolate was identified by a bacteriophage-typing pattern based on the lytic reaction of isolates to a number of bacteriophages;All bacteriophages showed similar morphology to Escherichia coli bacteriophage T[subscript]4 (Bradley's morphological group 'A') when examined by using transmission electron microscopy (TEM). Genetic variations were observed among the viral particles when endonuclease restriction patterns of genomic DNAs were compared. Lambda/Hind III digestion comparison with genomic DNAs resulted in approximately 50 kilobase pairs (kb) of bacteriophage DNA;All 37 serocluster 123 isolates were tested for their effectivity and their competitiveness in the greenhouse. In a screening experiment, the isolates ranged from high to low effectiveness in N[subscript]2 fixation, based on plant dry weight production, when tested on the cultivar 'Williams 82'. Eight isolates selected from both high and low fixation spectrums were tested on cultivars 'Corsoy 79' and Williams 82 in the greenhouse. Effectivity results from two trials and a third competition study were not consistent with the original experiment. Variability of effectivity and competition studies and a highly significant strain-by-cultivar interaction, may indicate the heterogeneous nature of serocluster 123. In general, strain S2.6 was found to be a competitive strain when challenged with antibiotic-marked strain I110 ARS on both cultivars (56.3% and 50.0% nodule occupancy on Corsoy 79 and Williams 82, respectively). This strain, however, tended to produce low yields on Williams 82 in three of the four experiments;Data suggest that natural variation exists and there may be potential for the selection of a native serocluster 123 isolate to be used commercially as an inoculum.