A field study on the competion and on the dynamics of occupation of nodulation sites by naturalized and seed inoculated strains of Bradyrhizobium japonicum was conducted on a Dark Red Latosol. The experimental design consisted of randomized blocks, with eight replications of the following treatments: A) control without inoculation; B) control without inoculation, with nitrogen fertilizer; C) inoculation with about 107 rhizobia/g (8g/kg of seed); D) inoculation with about 1010 rhizobia/g (8g/kg of seed). The inoculant was prepared with the strains SMS-314 (= SEMIA-587) and SMS-463 (=SEMIA-5019 = 29W), officially recommended for inoculant production. The identification of the strains which formed nodules was made in samples collected at 10, 42 and 70 days after seed germination. The grains were harvested to evaluate production and nitrogen content. Sorological typification was made with antiserum prepared from antigens constituted by B. japonicum strains SEMIA-587, SEMIA-5019 and SEMIA-5052 (=USDA-6), using the immunodot method. The characterization of the strains in nodules revealed a small increase (6,2 to 8,7%) in the occurrence of nodules formed by the recommended strains for the three sampling dates. There was an expressive participation of the strain 5052 (serogroup USDA-6) in the formation of the nodules. It was also observed that the naturalized strains may contribute differently to the formation of the nodules during the cycle, depending on the time of sampling.