The hydrolysis of p-nitrophenyl benzoate catalyzed by alpha-chymotrypsin in the presence of cetyltriphenylphosphonium bromide, cetyltributylphosphonium bromide and cetyltrimethylammonium bromide (pre and post micellar regions) has been studied. The ester is hydrolyzed readily by alpha-chymotrypsin in all the surfactants with the highest activity shown in cetyltributylphosphonium bromide. The dependences of the Michaelis constant and the catalytic constant with surfactant concentration have also been discussed.