Cyclopiazonic acid is the main toxic principle of a strain of Penicillium cyclopium Westling. On the basis of chemical and spectrochemical evidence it has been deduced that cyclopiazonic acid has the structure and relative stereochemistry shown in I. Penicillium cyclopium Westling has a world-wide distribution and is frequently isolated from stored grain and cereal products. Several strains of this species, recovered from samples of domestic cereal products, were found to cause acute toxicoses in ducklings and rats. Maize-meal was used for the large-scale cultivation of the most toxic strain and the toxic principles were extracted quantitatively with chloroform-methanol. From the fraction soluble in sodium hydrogen carbonate solution, cyclopiazonic acid (I) was isolated by chromatography on formamide-impregnated cellulose and ion-exchange (Dowex I) columns. This compound was found to be the main cause of the toxicity of the fungus.