The immobilized Candida antarctica lipase, a commercially available and one of the most commonly used enzymes, showed significantly improved enantioselectivity (twice) when used in a very acidic environment (pH 1.0) than in the normal pH 7.0 for the hydrolysis of ketoprofen ethyl ester at 45 oC. The enzyme was still 60% active at pH 1.0 compared to the activity at pH 7.0 and its stabilities at the two pH values were almost the same. The improved enantioselectivity was ascribed to the conformational change of the enzyme in the very acidic environment.