The infrared absorption and 1H nuclear magnetic resonance analyses of chloroform solutions of the terminally-blocked segment corresponding to the 2-9 sequence of emerimicins III and IV, -(Aib)3-L-Val-Gly-L-Leu-(Aib)2-, are consistent with the presence of a 3(10)-helical structure of high thermal stability. The crystal structure of the octapeptide, obtained by X-ray diffraction indicates the formation of a right-handed 3(10)-helix, stabilized by six consecutive intramolecular N-H....O:C H-bonds, slightly distorted at the level of the L-Leu residue.