Pilot-scale continuous mix bioreactors were used to study the effect of micro-aeration (0.00-0.15 vvm) on the cell growth of Lactobacillus helveticus, lactose utilization and lactic acid yield under batch fermentation of cheese whey having initial lactose concentrations ranging from 50 to 150 g/l. The experiments were conducted under controlled pH (5.5) and temperature (42 degrees C) conditions. The results indicated that micro-aeration significantly improved the maximum cell number and the specific growth rate. It also enhanced the lactose utilization rate and lactic acid yield. The optimum micro-aeration level for the lactose fermentation process was found to be 0.10 vvm. Further increases in the micro-aeration rate slightly decreased both the cell number and the lactic acid yield due oxygen toxicity to the bacterial cells. The maximum lactic acid yield was obtained with the micro-aeration rate of 0.10 vvm and the 75 g/l initial lactose concentration.