Four pilot-scale batch reactors of 5 l volume each were used to study the effect of micro-aeration (240-720 ml/min or 0.05-0.15 v/v/min) on the cell growth rate, substrate utilization and ethanol production during cheese whey batch fermentation using the yeast Candida pseudotropicalis at lactose concentrations ranging from 50 to 200 g/l. The results indicated that micro-aeration had a significant effect on the maximum cell concentration and the specific growth rate. It improved the viability of the yeast cells and enhanced the substrate utilization rate and the ethanol yield. The optimum micro-aeration level was found to be 0.1 v/v/min; further increases in the rate of micro-aeration resulted in an increased biomass concentration and decreased ethanol production as more lactose was utilized for biomass synthesis. The maximum ethanol yield (98.3%) was obtained with the micro-aeration rate of 0.1 v/v/min and the 150 g/l initial substrate concentration. The ethanol productivity (g/l/h) under these conditions was 35% higher than that obtained at 150 g/l initial substrate concentration with no aeration.