The catabolic ${alpha}-acetolactate$ synthase purified from Serratia marcescens ATCC 25419 was rapidly inactivated by the tryptophane-specific reagent, N -bromosuccinimide, and the arginine-specific reagent, phenylglyoxal. The enzyme was inactivated slowly by the cysteine-specific reagent N-ethylmaleimide. The second-order rate constants for the inactivation by N-bromosuccinimide, phenylglyoxal. and N -ethylmaleimide were $114,749M^{-1}min^{-1}$, $304.3M^{-1}min^{-1}$, and $5.1M^{-1}min^{-1}$, respectively. The reaction order with respect to N-bromosuccinimide, phenylglyoxal, and N-ethylmaleimide were 1.5,0.71, and 0.86, respectively. The inactivation of the catabolic aacetolactate synthase by these modifying reagents was protected by pyruvate. These results suggest that essential tryptophane, arginine, and cysteine residues are located at or near the active site of the catabolic ${alpha}-acetolactate$ synthase.