Surface tension measurement of the critical micelle concentration, CMC, of cationic hexadecyltrimethylammonium bromide, ctab, shows a significant increase on addition of very small amounts of 4- nitroacetanilide. At concentrations above the CMC, ctab catalyses the hydrolysis of 4-nitroacetanilide.The micellar catalysis is inhibited by added salts, especially those of arenesulfonates and bulky carboxylate ions. The kinetics have been analysed in terms of competitive binding of the substrate and the inhibitor to the micelle.