The relationships between the nitrate, nitrite, and oxygen respiratory pathways in Achromobacter fischeri have been studied. The formation of both nitrate and nitrite reductases is inhibited by oxygen. Nitrate and nitrite reductions by the growing cells occur only when the respiratory enzymes are not saturated with oxygen. Nitrite reduction also does not occur so long as there is nitrate in the growth medium. Nitrate is reduced to nitrite which is then reduced to ammonia. The reduction of nitrite with NADH or NADPH requires two enzymes: (a) NAD(P)H – flavin reductase, (b) nitrite reductase, plus the necessary participation of flavin as electron carrier. NAD(P)H – flavin reductase is required only to transfer electrons from NADH or NADPH to flavin, which then transfers electrons to nitrite reductase. The nitrite reductase from A. fischeri is nonspecific with respect to electron donor since it can use either FMNH2 or FADH2. The flavin reductase is also nonspecific with respect to electron donor as it can use both NADH and NADPH.Nitrate metabolism in A. fischeri is unique in that the product of nitrate metabolism is ammonia, but it resembles the respiratory type in other respects.