Ethanol fermentation of cheese whey powder (CWP) solution was realized in a fed-batch operated fermenter using the pure culture of Kluyveromyces marxianus (DSMZ 7239) at different feed CWP concentrations varying between 51 and 408 g l(-1). Total sugar content of the feed varied between 25 and 200 g l(-1) with a constant flow rate of 0.0841 h(-1). Sugar utilization, ethanol formation and biomass growth were investigated as function of the feed sugar concentration in a five-cycle repeated fed-batch operation where the system reached the quasi steady-state. Variations of the growth and the product yield coefficients with the feed sugar concentration were quantified. The growth yield coefficient (Y-X/S) decreased with increasing feed sugar concentrations. The product yield coefficient (Y-P/S) was almost constant (0.54 +/- 0.02 g EtOH g(-1) S) for sugar concentrations between 25 and 150 gl(-1), but decreased at the feed sugar concentration of 200 gl(-1) due to high osmotic pressure at high sugar concentrations. The highest ethanol concentration (63 g l(-1)) and the productivity (5.3 g EtOH h(-1)) were obtained with a 125 gl(-1) feed sugar concentration and sugar loading rate of 10.5 g S h(-1). (c) 2006 Elsevier Inc. All rights reserved.