SUMMARY Lipid contents of yeasts grown on glucose were: Candida lipolytica, 5.4%; C. tropicalis, 9'4 %; C. utilis, 2.7 yo; Candida 107, 41 %; Hansenula anomala, 12-5 %; Rhodotorula glutinis, 2.7 % ; and R. graminis, 9-1 %. In each yeast about 80 % of the lipid consisted of triglycerides. When the triglycerides from five of the yeasts were analysed in detail, an unsaturated acid was invariably found at the 2-position. With Candida 107 and R. graminis about 50 % of the total triglyceride fatty acids were saturated, resulting in over 50 % of the triglycerides being of the ~~-disaturated- 2-monounsaturated type. When Candida 107 and C. tropicalis were grown on individual n-alkanes, from C12 to CI6, the fatty-acid composition varied according to the chain length of the substrate, although with n-tridecane neither yeast produced tridecanoic acid in the triglyceride and with n-dodecane only C. tropicalis contained an appreciable amount of dodecanoic acidin the triglyceride (32 % of the fatty acids). With both yeasts on each alkane substrate, the lipid contents were not only lower than when grown on glucose but contained a smaller proportion of triglyceride. Saturated acids were now located at the 2-position of the triglycerides: Candida 107 grown on n-tetradecane produced 46 % of its triglycerides with a saturated acid at the 2-position. The main advantage to be gained by growing yeasts on n-alkanes is, as far as lipid formation is concerned, the biosynthesis of specific fatty acids rather than the production of plant-like triglycerides.