The effect of the Bradyrhizobium japonicum hydrogenase on nitrogen fixation was evaluated by comparing the growth of Vigna and Glycine species inoculated with a Hup mutant and its Hup+ revertant. In all experiments, the growth of plants inoculated with the strain without hydrogenase was at least equal to the growth of the strain with hydrogenase. For Glycine usuriensis and Glycine max cv. Hodgson in liquid culture, the growth was higher with the Hup− strain. It is possible that reduced rates of nitrogen fixation in the presence of hydrogenase are due to O2 depletion caused by the hydrogen oxidizing, since the oxygen pressure in the air appears to be a limiting factor of symbiotic nitrogen fixation in the soybean.