A single-crystal x-ray diffraction analysis of Boc-L-Ala-D-aIle-L-Ile-OMe has been carried out. The analysis has shown (a) that the tripeptide molecules have in part an alpha-extended conformation, the torsion angles of the L-Ala and D-aIle residues being psi 1 = -75.1 degrees and psi 1 = -25.8 degrees and psi 2 = 67.3 degrees and psi 2 = 44.1 degrees, respectively, and (b) that the molecules are organized in rippled planes where they occur in relative antiparallel orientation linked together side by side by H bonds. This molecular organization of the tripeptide corresponds closely to that of an antiparallel alpha-pleated sheet, and likely constitutes the first example of a structure of this kind for which a characterization at the atomic level has been achieved. A molecular dynamics study has shown that the molecular conformation of the tripeptide in the crystalline state is determined primarily by intermolecular interactions.