The selective preparation of the four stereoisomers of beta -methylphenylalanine (Mphe) from mixtures of the four stereoisomers of N-carbamoyl- beta -methylphenylalanine (NCMphe) with N-carbamoyl amino acid amidohydrolases (carbamoylases) was developed. D-Carbamoylase specifically hydrolyzed threo-D-NCMphe with a little side activity toward erythro-D-NCMphe, thus threo-D-Mphe was produced with high optical purity from a mixture of the four stereoisomers of NCMphe. L-Carbamoylase specifically produced threo-L-Mphe from a mixture of the four stereoisomers of NCMphe. The erythro-D-Mphe was obtained from erythro-DL-NCMphe which was prepared through diastereomer resolution by separative crystallization of benzoyl Mphe with a little side activity of D-carbamoylase toward erythro-D-NCRMphe and the remaining erythro-L-NCMphe was chemically hydrolyzed to erythro-L-Mphe. (C) 2001 Elsevier Science B.V. All rights reserved.