Biomass yields for several null mutants in Saccharomyces cerevisiae were successfully predicted with a metabolic network model. Energetic parameters of the model were obtained from growth data in C-limited aerobic chemostat cultures of the corresponding wild-type strain, which exhibited a P/O ratio of 1.46, a non-growth-related maintenance of 56 mmol ATP/C-mol biomass/h, and a growth-related requirement of 655 mmol ATP/C-mol biomass. Biomass yields and carbon uptake rates were modeled for different mutants incapacitated in their glyoxylate cycle and their gluconeogenesis. Biomass yields were calculated for different feed ratios of glucose to ethanol, and decreases for higher ethanol fractions were correctly predicted for mutants with deletions of the malate synthase, the isocitrate lyase, or the phosphoenolpyruvate carboxykinase. The growth of the fructose- 1,6-bisphosphatase deletion mutant was anticipated less accurate, but the tendency was modeled correctly.