Helminthosporium carbonum, which is pathogenic to maize, produces a metabolite, HC-toxin (I), with toxicity to susceptible host genotypes. Resistant genotypes and non-host plants are tolerant. The structure of (I) was determined as cyclo[(2-amino-9,10-epoxy-8-oxodecanoyl)-L-alanyl-L-alanyl-prolyl] based upon spectral evidence. Helminthosporium carbonum Ullstrup race 1 selectively parasitizes certain genotypes of maize causing necrotic lesions on the leaves. Scheffer and Ullstrup found that the fungus produces a toxic substance named HC-toxin which has the same specificity for maize genotypes as does the producing fungus. Sensitive tissue is affected by the toxin at very low concentrations (0.1 μg/ml), whereas resistant plants will tolerate much higher concentrations with no obvious damage. The toxin was shown to be required for successful colonization of susceptible plants. HC-toxin causes various physiological changes in sensitive cells, including an immediate but temporary increase in negative electropotential across the cell membrane, an increase in dark fixation of CO₂, and an increase in the uptake of nitrate and other solutes. Eventually, exposure to HC-toxin leads to an increase in tissue respiration and loss of materials from the cell. HC-toxin appears to have a primary effect on the plasma membrane. HC-toxin previously was isolated from culture filtrates and partially characterized. Pringle reported that the toxin has a molecular formula C₃₂H₅₀N₆O₁₀ based upon elemental analysis, and did not react with ninhydrin. Alanine and proline in a 2 to 1 ratio were found among the hydrolytic products and a third ninhydrin-positive component was detected. A structure was not proposed. Several peptidic fungal products affecting higher plants have been reported. The host selective toxins from Alternaria mali affecting apple are closely related cyclic tetra-depsipeptides. Cyclic tetrapeptides include tentoxin from Alternaria alternata f. tenuis, and Cyl-2 from Cylindrocladium scoparium Morgan and chlamydocin from Diheterospora chlamydospora which both contain the novel amino acid 2-amino-9,10-epoxy-8-oxodecanoic acid. In this paper, we report the structure of HC-toxin (I), which is a cyclic tetrapeptide of the same structural family as Cyl-2 and chlamydocin.