The title amidino-amino acids (a-Hpro), C6H11N3O3.H2O, (I), and a-Met, C6H13N3O2S.H2O, (II), respectively, exist in the form of zwitterions. The five-membered pyrrolidine ring in (I) adopts an envelope conformation, with the Cgamma atom out of the plane defined by the rest of the ring atoms, and with the hydroxyl and carboxylate groups in a trans configuration relative to the ring plane. The two crystallographically independent zwitterions in (II) reveal quite different conformations of their side chains and a slightly different orientation of the guanidine moiety with respect to the carboxylate group. The crystal structures of both (I) and (II) are stabilized by extensive networks of O-H...O, N-H...O and C-H...O hydrogen bonds, the network being three-dimensional in (I) and two-dimensional in (II).