The effects of bile salt cosurfactants on the lipase-catalyzed hydrolysis of p-nitrophenyl palmitate (p-npp) in AOT reversed micelles art described. Both an anionic bile salt, sodium taurocholate (NaTC), and a zwitterionic bile salt, 3-[(3-cholamidyl-propyl)dimethylammonio] (CHAPS), increase the reaction velocity of the hydrolysis reaction and the stability of the enzyme in the reversed micelles. CHAPS provides significantly larger increases in activity and stability than NaTC. Fluorescence studies indicate that addition of bile salt serves to immobilize the protein at the detergent layer and decrease the polarity experienced by the protein at the interface. Comparison of the results for lipase with previous results for other enzymes in the AOT/bile salt reversed micellar system lead us to conclude that the mechanisms by which bile salts affect enzymatic activity and stability in AOT reversed micelles vary from protein to protein. The effects, which generally are positive (increasing activity and stability), are greatest for lipase, which operates at organic/water interfaces and is known to interact with bile salts in physiological systems.