The rate of hydrolysis of 2-naphthyl acetate catalyzed by alpha-chymotrypsin has been measured in aqueous solutions of dodecyltrimethylammonium bromide at concentrations below and above the critical micelle concentration, as well as in the absence of surfactant. Under all the conditions employed, the reaction takes place following a Michaelis-Menten mechanism. The presence of the surfactant, at concentrations above its critical micellar concentration, increases the value of the Michaelis constant, K-m, without significant changes in the catalytic rate constant, k(cat). The increase in K-m is larger than that expected from the incorporation of the substrate to the micellar pseudophase, indicating that the interaction between the enzyme and micellelike aggregates alters the formation of the enzyme-substrate complexes. This can be related to a partial unfolding of the enzyme, as is suggested by changes in its intrinsic fluorescence.