The solution and solid structure of Boc-L-Val-Delta Phe-Delta Phe-Delta Phe-L-Val-OMe, containing three consecutive Delta Phe residues, have been determined by X-ray diffraction, nuclear magnetic resonance, and circular dichroism methods. The crystals grown from aqueous methanol are orthorhombic, space group P2(1)2(1)2(1), a = 11.624(2), b = 17.248(2), c = 21.532 Angstrom, V = 4216 (1) Angstrom(3), Z = 4. In the solid state, the peptide exhibits a left-handed 3(10)-helical conformation, in spite of the presence of two L-Val residues. NMR and CD studies in different solvents also support the crystal structure data, suggesting that the solid state structure is maintained in solution as well. This is the first report of a dehydropeptide containing three consecutive Delta Phe residues and exhibiting left-handed 3(10)-helical conformation, which demonstrates the remarkable conformational consequences produced by consecutive occurrence of Delta Phe residues in a peptide.