Abstract 1. Growth yields of Saccharomyces cerevisiae grown in complex media with glucose, ethanol, d- and l-lactates, pyruvate or acetate, respectively, as the energy source were measured. The cell mass formed per mole of ATP which would have been produced from the substrates according to theoretical stoichiometry, was determined. Assuming that on the average 10.5 g dry wt. of cells were formed per mole ATP (the Elsden constant), the amount of ATP furnished by fermentation and oxidation of glucose and by the oxidation of the other substrates by growing cells was calculated. 2. Phosphorylation efficiencies in growing respiration-deficient and oxidative phosphorylation-deficient mutants were evaluated from growth yield measurements. 3. The effects of antimycin A, 2,4-dinitrophenol, oligomycin and azide on phosphorylation efficiencies in growing cells were examined. 4. The results suggest that in yeast growing in a complex medium on glucose and lactate, energy-producing and energy-consuming reactions are effectively coupled. The lack of the first oxidative phosphorylation site in yeast is corroborated. Functioning of the antimycin-binding site and equilibration of cellular ATP with mitochondrial high-energy state do not seem to be obligatory for anaerobically growing yeast.