A novel peptide, XR586, has been isolated from fermentations of Acremonium persicinum (Xenova culture collection number X21488). The structure of XR586 has been elucidated by means of NMR spectroscopy, electrospray and fast-atom bombardment MS, derivatization and enzymic digestion. It has been shown to be helical by CD measurements. XR586 shows many structural and conformational features in common with peptaibols, particularly the zervamicins. Peptaibol antibiotics are peptides, typically of 15-20 residues, containing a large proportion of alpha-aminoisobutyric acid (Aib) residues. These peptides adopt a helical conformation in solution and display anti-bacterial and toxic properties due to their ability to form pores in membranes. However, while XR586 contains several Aib residues, it lacks a terminal phenylalaninol and terminates in the sequence Phe-Gly. The lack of reduction of the penultimate residue at the C-terminus may indicate that this step is normally at the end of the biosynthetic pathway of peptaibols and occurs with cleavage of Gly. The 1H chemical shift assignments of XR586 are reported in Supplementary Publication SUP 50179 (3 pages), which has been deposited at the British Library Document Supply Centre, Boston Spa, Wetherby, West Yorkshire LS23 7BQ, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1996) 313, 9 ("Deposition of data').