The invention concerns a method for preparing .alpha.-phenylalanine ester with substantially enantiomeric purity of formulae (I) and (II) wherein Alk represents a linear or branched C.sub.1 -C.sub.6 alkyl radical, characterized in that it consists in contacting a mixture of .alpha.-phenylalanine ester enantiomers with a lipase, a protease or an esterase. ##STR1##