An alternative method for the conversion of cheese whey lactose into ethanol has been demonstrated. With the help of continuous-culture technology, a catabolite repression-resistant mutant of Saccharomyces cerevisiae completely fermented equimolar mixtures of glucose and galactose into ethanol. The first step in this process was a computer-controlled fed-batch operation based on the carbon dioxide evolution rate of the culture. In the absence of inhibitory ethanol concentrations, this step allowed us to obtain high biomass concentrations before continuous fermentation. The continuous anaerobic process successfully incorporated a cell-recycle system to optimize the fermentor productivity. Under conditions permitting a low residual sugar concentration (≤1%), maximum productivity (13.6 g liter⁻¹ h⁻¹) was gained from 15% substrate in the continuous feed at a dilution rate of 0.2 h⁻¹. Complete fermentation of highly concentrated feed solutions (20%) was also demonstrated, but only with greatly diminished fermentor productivity (5.5 g liter⁻¹ h⁻¹).