The conformation of the benzodiazepine-like decapeptide, YLGYLEQLLR, corresponding to residues 91–100 of bovine αs1-casein, has been examined in SDS micelles using CD, two-dimensional 1H-NMR and restrained molecular-dynamics simulation. Evidence is presented that the decapeptide adopts a rigid structure in water/SDS micellar medium, but not in water or dimethylsulfoxide. The three-dimensional structure, consistent with the proton-proton distances obtained from the quantitative analysis of the two-dimensional NOES, was generated by restrained energy minimization and molecular-dynamics simulation. In water/SDS micellar medium, YLCYLEQLLR adopts an amphipathic helicoid structure with distinct hydrophobic and hydrophilic faces. The relative disposition of the tyrosine aromatic rings was compared with that of the aromatic rings in the benzodiazepines.