Harzianin HK VI, one of the major peptides of the natural mixture of peptaibol antibiotics biosynthesized by the fungus Trichoderma pseudokoningii, has been isolated by a multi-step chromatography procedure including HPLC. Its sequence was determined by positive ion FAB MS and two-dimensional ¹H and ¹³C NMR experiments (COSY, HOHAHA, HSQC-HOHAHA, HMBC), allowing the location in the sequence of two isoleucines at positions 3 and 4 and of two leucines at positions 7 and 8, which could not be distinguished on the basis of their ¹H NMR spin systems. Harzianin HK VI is an 11-residue peptaibol with two Aib-Pro sequences at positions 5-6 and 9-10. From ROESY data and amide temperature coefficients, a β-bend ribbon stabilised by intramolecular hydrogen bonds of the 4+1 type is suggested. Despite the short sequence, harzianin HK VI increases the permeability of liposome bilayers to the same extent as longer 14-residue peptaibols.