The primary structure of HC-toxin, a host-selective plant toxin produced by the fungus Helminthosporium carbonum, has been determined as a cyclic tetrapeptide. In addition to an uncommon α-amino acid, 2-amino-8-oxo-9,10-epoxydecanoic acid, the toxin contains one proline and two alanine residues. The sequence has been established as cyclo- [prolylalanylalanyl-2-amino-8-oxo-9,10-epoxydecanoyl].