<jats:p><jats:bold>The Structure of Cyclosporin C</jats:bold><jats:p>The structure of cyclosporin C (<jats:bold>2</jats:bold>), a minor antifungal metabolite from <jats:italic>Trichoderma polysporum</jats:italic> (Link ex Pers.) R<jats:sc>IFAI</jats:sc> has been elucidated. Hydrolytic cleavage and spectroscopic evidence show that cyclosporin C is a neutral oligopeptide of 11 amino acids linked together in a 33‐membered ring. Cyclosporin C (<jats:bold>2</jats:bold>) differs from the main metabolite cyclosporin A (<jats:bold>1</jats:bold>) [2] [4] only by containing L‐threonine in the place of L‐α‐aminobutyric acid as has been shown by the conversion of <jats:bold>2</jats:bold> into <jats:bold>1</jats:bold>. <jats:sup>13</jats:sup>C‐NMR. spectra and study of molecular models suggest that cyclosporin C (<jats:bold>2</jats:bold>) has the same molecular conformation as <jats:bold>1</jats:bold>, which is best described as a twisted β‐pleated sheet held together in a conformationally stable form by intramolecular hydrogen bonding.