Enzymatic activity of chymotrypsin in AOT reversed micelles is facilitated by the addition of a bile salt cosurfactant, sodium taurocholate (NaTC). NaTC diversifies the interfacial properties of the reversed micelles and increases their water capacity, resulting in a more favorable environment for enzymatic catalysis. The reaction velocity for the hydrolysis of the substrate N-glutaryl-L-phenylalanine p-nitroanilide (N-GPNA) by chymotrypsin more than doubles when NaTC is added to AOT reversed micelles in heptane. The enzymatic reaction obeys Michaelis-Menten kinetics in AOT/heptane reversed micelles over the range of NaTC concentrations studied and within a concentration range of 0.05-0.30 mM N-GPNA. NaTC causes changes in the enzyme turnover number, k(cat), the Michaelis constant, K-M, and the catalytic efficiency of the enzyme, k(cat)/K-M, that are generally consistent with increased enzymatic activity. Similar effects are seen in dodecane, suggesting that exchange of reactants and products among aqueous pools is not a rate-limiting factor in this system.