The activity of nitrate reductase in Rhizobium japonicum is controlled by oxygen tension, and not by nitrate. The enzyme from R. japonicum grown anaerobically in the presence of nitrate resembles that from bacteroids in having a molecular weight of about 69000 daltons; the enzyme from aerobically grown cells has a molecular weight of about 170000 daltons. Both types of enzyme have similar Km values, but differ in their sensitivity to KCN.