Based on experimental results and the results presented in the literature a model for the meta bolism of baker's yeast under biotin deficiency is presented. In these conditions the most sensi tive point in the metabolism is the carboxylation of pyruvate to oxaloacetate catalyzed by the biotin-containing pyruvate carboxylase. Because the rate of glycolysis is not affected by biotindeficiency pyruvate accumulates in the cells and is partially excreted into the medium. The high pyruvate pool in the cells means that the metabolism is mainly fermentative even in vigorously aerated cultures. The oxidation of pyruvate to acetyl-CoA proceeds almost unaffected, as can be seen by the production of elevated amounts of ethyl acetate by yeast grown under sub-optimal biotin concentrations. Acetyl-CoA carboxylase, which also has biotin as the prosthetic group, is not as sensitive to a deficiency of this growth factor as is pyruvate carboxylase. In yeast grown without biotin the amounts of fatty acids and lipids are the same or even higher than in cells grown under optimal conditions. The metabolism from oxaloacetate towards the TCA cycle and glutamate is not affected as strongly as is the metabolism to aspartate, which is present in cells in strictly limited amounts. This causes an over-production of metabolic intermediates, e.g. diazotizable arylamine and hypoxanthine as well as citrulline. Their conversion to normal cell constituents, purine nucleotides and arginine, is dependent on the aspartate available and is thus depressed by lack of biotin.