The interaction of a series of aromatic dyes with the coenzyme A binding site of choline acetyltransferase was studied. Several of the dyes were very potent inhibitors of the enzyme. With few exceptions, inhibition was competitive with respect to acetylcoenzyme A and noncompetitive with respect to choline. It appears likely that inhibition by dyes such as Reactive Blue 2 (Cibacron Blue F3GA) or Congo Red, as in the case of coenzyme A interactions, involves hydrophobic bonding, as well as a coulombic interaction with an arginine residue.