About a dozen different fluorescent substances have been isolated from cultures of Aspergillus flavus (Link) in natural and synthetic media by Smith and McKernam [1] and by Asao, Buchi et al. [2]. Only the aflatoxins B₁, B₂, G₁, G₂ and three related quinones have been reported as being toxic. We therefore decided to investigate the nature and toxic properties of two substances previously referred to as fractions 10 and 11 by Smith and McKernam [1]. It is shown in this report that these substances may be purified as crystalline, unsaturated fatty acids. The substance B₀ which gives a blue fluorescence is as toxic to chick embryo as aflatoxin B₁ while that which gives a green fluorescence in ultraviolet light (G₀) is as toxic as aflatoxin G₂. Structural analyses (including thin layer chromatography, mass spectrometry, infrared, ultraviolet spectra, nuclear magnetic resonance, etc.) lead to the conclusion that B₀ and G₀ are non-aromatic but probably spatial isomers of hydroxy-conjugated fatty acids (probably diene acids) with the molecular formula C₂₄H₄₄O₃ (corrected from C₂₄H₄₂O₃ based on mass ion peak of 380). Toxicity titration of 6-day old White Rock chick embryos shows B₀ to be as toxic as aflatoxin B₁ while G₀ is as toxic as aflatoxin G₂.