Summary: Effects of oxygen upon derepression of nitrogenase were studied in Klebsiella pneumoniae, using oxyleghaemoglobin to supply and monitor very low dissolved O2 concentrations in a steady-state system. Expression of the nifH gene was studied by using a nifH::lac fusion strain, which also carried the Nif+ plasmid pRD 1 so that the production of active nitrogenase could also be monitored. When compared with anaerobic treatments, very low concentrations of dissolved O2 inhibited derepression of both nifH::lac and pRD1 nif. Fifty percent inhibition of derepression occurred at 0.1 μm-O2. The apparent KS of the dominant terminal oxidase was 0.08 μm-O2. These results suggest that there is a close relationship between the terminal respiratory system of these bacteria and the repression of nitrogenase by O2.