A potent denitrifying bacterium, Alcaligenes faecalis S-6, was isolated from activated sludge, and the regulatory mechanisms for dissimilatory nitrite reductase (NIR) were studied. NIR activity was controlled by oxygen and nitrite through dual regulatory mechanisms, i.e., induced synthesis and inactivation of .the enzyme. Full induction of NIR synthesis required both low oxygen tension and the presence of nitrate or nitrite as inducer. Nitrite seemed to be the real inducer, since nitrate failed to induce synthesis with a nitrate reductase deficient mutant. Intracellular NIR was inactivated by oxygen, and the inactivation was stimulated by cysteine and prevented by nitrite. NIR in the crude cell free extract was inactivated by oxygen when cysteine was added, but that in the partially purified preparation was not. An inactivating factor which may possibly be a protein catalyzing the inactivation was found in the cell extract.