AbstractThe production of ethanol from cheese whey lactose has been demonstrated using a single‐stage continuous culture fermentation with 100% cell recycle. In a two‐step process, an aerobic fed batch operation was used initially to allow biomass buildup in the absence of inhibitory ethanol concentrations. In the anaerobic ethanol‐producing second step, a strain ofKluyveromyces fragilisselected on the basis of batch fermentation data had a maximum productivity of 7.1 g ethanol/L/h at a dilution rate of 0.15 h−1, while achieving the goal of zero residual sugar concentration. The fermentation productivity diminished when the feed sugar concentation exceeded 120 g/L despite the inclusion of a lipid mixture previous shown to enhance batch fermentation product