The tetrapeptide t-butyloxycarbonyl-α-aminoisobutyryl-α-aminoisobutyryl-L- phenylalanyl-L-methionyl amide crystallizes in the orthorhombic space group P2₁2₁2₁ with a= 9.096, b= 18.067, c= 21.701 Å and Z= 4. The crystals contain one molecule of dimethyl sulphoxide (DMSO) associated with each peptide. The structure has been solved by direct methods and refined to an R value of 0.103 for 2 672 observed reflections. The peptide adopts a distorted 3₁₀ helical structure stabilized by two intramolecular 4 → 1 hydrogen bonds between the Boc CO and Aib(1) CO groups and the NH groups of Phe(3) and Met(4), respectively. A long hydrogen bond (N ⋯ O = 3.35 Å) is also observed between Aib(2) CO and one of the terminal amide hydrogens. The DMSO molecule is strongly hydrogen bonded to the Aib(1) NH group. The solid-state conformation agrees well with proposals made on the basis of n.m.r. studies in solution.