Alpha-aminoisobutyric acid-directed biosynthesis in two Trichoderma strains has been shown to lead to the simplification of the natural peptaibol microheterogeneous mixtures and to the production of new analogues. Hence, two new peptides originating from T. harzianum, trichorzin PA(U) 4 and harzianin PCU 4, were isolated by HPLC. Their sequences were determined by positive liquid secondary-ion mass spectrometry (LSI MS). Trichorzin PA(U) 4 and harzianin PC(U) 4 are 18- and 14-residue peptaibols, respectively, both containing a high proportion of alpha-aminoisobutyric acid (Aib). LSI MS performed with lithium cationized peptides, allowed to assign the relative position of leucine/isoleucine isomeric residues, even without the use of tandem mass spectrometry.