The fixed oil of the poisonous Jack O'Lantern mushroom has been analyzed, and the usual physical and chemical constants for fats have been determined. The fatty acids, as methyl esters, were resolved by gas chromatography using a liquid phase of 25 per cent diethylenegly colsuccinate polyester and 2 per cent phosphoric acid on Chromosorb W and a thermal conductivity detector. The fatty acids present in the oil include myristic, myristoleic, palmitic, palmitoleic, stearic, oleic, and linoleic acids. The unsaponifiable fraction was analyzed by gas chromatography using a liquid phase of 0.4 per cent silicone gum rubber on Chromosorb W and a hydrogen flame ionization detector. Ergosterol accounted for 76.2 per cent of this fraction. The fixed oil of the poisonous Jack O'Lantern mushroom has been analyzed, and the usual physical and chemical constants for fats have been determined. The fatty acids, as methyl esters, were resolved by gas chromatography using a liquid phase of 25 per cent diethylenegly colsuccinate polyester and 2 per cent phosphoric acid on Chromosorb W and a thermal conductivity detector. The fatty acids present in the oil include myristic, myristoleic, palmitic, palmitoleic, stearic, oleic, and linoleic acids. The unsaponifiable fraction was analyzed by gas chromatography using a liquid phase of 0.4 per cent silicone gum rubber on Chromosorb W and a hydrogen flame ionization detector. Ergosterol accounted for 76.2 per cent of this fraction.