The title compound, C 10 H 13 NO 2 ÁH 2 O, crystallizes in a zwitterionic form as a monohydrate, involving the propylbenzene group with a trans conformation. It is a non-natural amino acid, and has attracted attention as an inhibitor of phenylalanine hydroxylase. In the crystal, molecules are linked by N-HÁ Á ÁO hydrogen bonds, forming C(5) chains along the c-axis direction. Two chains are linked by another N-HÁ Á ÁO hydrogen bond, forming an R 3 3 (11) ring motif. Further O-HÁ Á ÁO hydrogen bonds link these motifs via the water molecules, to form a three-dimensional framework.