A 5 L continuous mix, batch bioreactor was used to investigate the effect of temperature on the growth of Lactobacillus helveticus and production of lactic acid from lactose. The temperature levels used were 23 degrees C (no control), 37 degrees C (no control), 37 degrees C (controlled) and 42 degrees C (controlled). No pH control was provided. The temperature and pH were monitored during the fermentation process. The pH steadily decreased from the initial value of 4.4 to less than 3.0, due to the lactic acid formation. Increasing the fermentation temperature from 23 degrees C to 42 degrees C (with no pH control), enhanced the lactose utilization and lactic acid production by 26.6 % and 6.2 g L-1, respectively. Maximum specific growth rate, lactose utilization and lactic acid production were 0.25 h(-1), 60.6 % (of initial concentration), and 10.0 g L-1 respectively, for fermentation with temperature control at 42 degrees C. The results showed the need for controling both temperature and pH during batch lactic acid fermentation from cheese whey to avoid yield losses.