Bacteroids isolated from nodules produced by one strain of Rhizobium japonicum (CC705) had strong nitrate-reducing activity and reduced C₂H₂ to C₂H₄ and N₂ to NH₃ anaerobically with nitrate. Bacteroids of another strain (CB1809) were much less active nitrate reducers and reduced little C₂H₂ anaerobically. Nitrite, which accumulated in the medium in anaerobic assays, was an inhibitor of C₂H₂reduction in both aerobic and anaerobic conditions. Succinate, at about 25 mM, stimulated both nitrate reduction and C₂H₂ reduction under aerobic conditions. Glucose stimulated C₂H₂ reduction up to 120 mM but nitrate reduction was inhibited in the presence of glucose. In terms of electrons transferred, the aerobic pathway appeared to be about 2.5 times more efficient than the anaerobic pathway in supporting nitrogenase activity of CC705 bacteroids.