Bradyrhizobium (Parasponia) sp. ANU 289 possessed an assimilatory nitrate reductase which was induced by nitrate and repressed by ammonia. Nitrate reductase was rapidly removed or inactivated as nitrate was depleted from the growth medium. Strain ANU 289 was able to take up nitrate only after growth on nitrate-containing medium and nitrate uptake was dependent on assimilation of nitrate by nitrate reductase. Two mutants affected in nitrate assimilation were isolated after transposon mutagenesis. One of these retained the uninduced level of nitrate reductase under all growth conditions, while the other showed a delay in nitrate reductase induction. Both mutants had a reduced ability to fix nitrogen symbiotically and in vitro. It is suggested that factors involved in molybdenum metabolism or synthesis of electron transport components, which are required for activity of both nitrogenase and nitrate reductase, were affected in the mutants.