The effects of oxygen, malonate, and acetate on the conversion of radioactively-labeled Coenzyme A thioesters of lauric and stearic acids to unsaturated fatty acids by 15,000 x g supernates of Penicilliumlehrysogenum were studied. Following the termination of reactions, the incubation products were saponified, acidified, extracted, and methylated. The resulting methyl esters were purified, separated, identified, and collected. Radioactivities of each fatty acid fraction were determined and converted to percentage of total radioactivity recovered.Desaturase activity was virtually non-existent in the anaerobic incubations indicating that this organism utilized the aerobic pathway of desaturation. The absence of acetate and malonate from incubation mixtures of either laurate or stearate markedly decreased the overall desaturase activity and the amount of linoleate produced. The failure of labeled laurate to be converted to long-chain unsaturated fatty acids in the absence of malonate indicates that chainelongation in this organism proceeds by the malonate pathway. The data presented suggest that laurate may be directly desaturated and then elongated to oleate instead of following the typical aerobic pathway. / Department of Biology