Cheese whey powder (CWP) solution with different CWP or sugar concentrations was fermented to ethanol in a continuous fermenter using pure culture of Kluyveromyces marxianus (DSMZ 7239). Sugar concentration of the feed CWP solution varied between 55 and 200 g l(-1) while the hydraulic residence time (HRT) was kept constant at 54h. Ethanol formation, sugar utilization and biomass formation were investigated as functions of the feed sugar concentration. Percent sugar utilization and biomass concentrations decreased and the effluent sugar concentration increased with increasing feed sugar concentrations especially for the feed sugar contents above 100 g l(-1). Ethanol concentration and productivity (DP) increased with increasing feed sugar up to 160 g l- 1 and then decreased with further increases in the feed sugar content. The highest ethanol concentration (3.7%, v v(-1)) and productivity (0.54 gEl(-1) h(-1)) were obtained with the feed sugar content of 100 g l(-1) or 125 g l(-1). The ethanol yield coefficient (Y-P/S) was also maximum (0.49 gE gS(-1)) when the feed sugar was between 100 and 125 g l(-1). The growth yield coefficient (Y-X/S) decreased steadily from 0.123 to 0.063 gX gS(-1) when the feed sugar increased from 55 to 200 g l(-1) due to adverse effects of high sugar contents on yeast growth. The optimal feed sugar concentration maximizing the ethanol productivity and sugar utilization was between 100 and 125 g l(-1) under the specified experimental conditions. (C) 2007 Elsevier Inc. All rights reserved.