Precise determination of yields deduced from measurements of metabolic rates needs an understanding of the physiology and energetics of growth of microorganisms. The linear growth equation derived from the simplified picture of microbial metabolism is generally accepted. However, the significance of the yields it gives is less well understood. The analysis of the theoretical and real observed and true yield coefficients for partly aerobic and anaerobic growth of S. cerevisiae is presented in this study. In the calculations of yields and maintenance coefficients, the pseudoconstants Y-ATP(X,Y-P),(max) P/O were used. A set of limitations imposed on true yields in the linear growth equation is established and the significance of the yields is also discussed. The experimental runs were performed in a laboratory-scale bioreactor (1.5 dm(3) of operating capacity). The yeast Saccharomyces cerevisiae hybrid G-67 was cultured in synthetic Reader medium containing sucrose as the main carbon and energy source. The comparison between experimentally obtained yield coefficients relative to cell growth and ethanol formation and the real theoretical values is presented. The maintenance coefficients calculated for anaerobic and respiro-fermentative processes are 0.14 C-mole ATP (C-mole DM h)(-1).