C21H21N3O5, Mr = 395.42, orthorhombic, P2(1)2(1)2(1), a = 5.935 (2), b = 13.037 (1), c = 25.992 (2) A, V = 2011 (1) A3, Z = 4, D chi = 1.31 Mg m-3, lambda(Cu K alpha) = 1.5418 A, mu = 0.74 mm-1, F(000) = 832, room temperature, final R = 0.046 for 1343 observed reflections. The title compound adopts a type II' beta-turn conformation in the solid state, stabilized by a 4----1 intramolecular hydrogen bond between the CO of the protecting benzyloxycarbonyl group and the NH2 of the terminal amide group. These results indicate that the beta-turn conformation, already found in the solid state for peptides embodying the sequence Asu-Gly or Asu-Ala, is highly preferred, even when the residue next to the aminosuccinyl moiety has a bulkier side chain. In the crystal, rows of hydrogen-bonded molecules are held together by van der Waals forces between hydrophobic phenyl groups.