The production of L-amino acid by the stereoselective hydrolysis of a D, L-amino acid ester by means of alpha-chymotrypsin was studied as a model of an enzymic reaction in an emulsion. Continuous operation involving the use of an emulsion of trioctylmethylammonium chloride/Span 80/kerosene, reported previously, was found to become unusable after nine hours because of swelling of the emulsion. To prepare a more stable emulsion, the various combinations of the membrane phase were examined. A mixture of equal weights of Span 80 and Paranox 100 gave the highest production rate and the most stable emulsion. The life of the emulsion in the present system was longer than that in the system reported previously.