For the molybdoenzymes synthesis as the nitrogenase, the molybdenum, in its most stable form, the molybdate, must be transported inside the cell. In Bradyrhizobium japonicum, the modABC genes code for a high-affinity ABC-type molybdate transporter. This work allowed to study the effect of inoculation of soybean plants with strains affected in the molybdate transport. modA and modB mutants, unable to grow in culture media under molybdate-deficient conditions, were used in our experiments. When soybean plants were inoculated with one of these strains and grown in a molybdate-deficient mineral solution did not affect the nodulation, but the nitrogen-fixing ability of the mod mutants was severely impaired. Addition of molybdate to the nutrient mineral solution used for plant growth fully restaured the wild-type phenotype, and the amount of molybdate required for supression of the mutant phenotype was dependent on sulfate concentration. Molybdate concentration required for the functioning of the mutant strains was greater when the medium was supplemented with high amounts of sulfate. Our results suggest the existence in B. japonicum, at least, three independent molybdate transport systems, a high-affinity transporter encoded by the modABC genes, a low affinity system corresponding to a sulfate transporter, and a third transporter that would be functional in the presence of high sulfate concentrations.