The enzyme activity and the oxidation state of soluble, activated, substrate-reduced succinate dehydrogenase are modified by the presence of bromide. The anion inhibits the enzyme by two different mechanisms which depend on the ratio of bromide to sucinate. At high ratios binding of two bromide ions is required and a competitive inhibition is observed: removal of succinate from the substrate binding site (Kd= 0.1mM) leads to oxidation of the flavin. At lower ratios but with sufficient succinate to saturate a site with Kd= 1.52 mM, unompetitive inhibition by a single bromide ion is observed. Mechanisms, as well as the possible physiological signifinance of the novel type of regulation of succinate dehydrogenase, are discussed.