A biochemically structured model has been developed to describe the continuous fermentation of lactose to ethanol by Kluveromyces marxianus and allowed metabolic coefficients to be determined. Anaerobic lactose-limited chemostat fermentations at different dilution rates (0.02 – 0.35 h-1) were performed. Species specific rates of consumption/formation, as well as yield coefficients were determined. Ethanol yield (0.655 C-mol ethanol*C-mol lactose-1) was as high as 98 % of theoretical. The modeling procedure allowed calculation of maintenance coefficients for lactose consumption and ethanol production of ms = 0.6029 and me= 0.4218 (C-mol)*(C-mol*h)-1, respectively. True yield coefficients for biomass, ethanol and glycerol production were calculated to be Ytrue sx = 0.114, Ytrue ex = 0.192 and Ysg = 2.250 (C-mol)*(C-mol)-1, respectively. Model calculated maintenance and true yield coefficients agreed very closely with those determined by regressions of the experimental data. The model developed provides a solid basis for the rational design of optimised fermentation of cheese whey.