A new parameter ethanol evolution rate (EER) was developed to aid in the determination of glucose feeding rate in avermectin production. The EER characterized the level of primary metabolism and its value was affected mainly by the supply of O2 and glucose. In an abnormal batch, over-feeding of glucose led to 2.5-fold increase of the maximum EER value compared to the normal one, and the production was thus decreased by nearly 80%. Together with other criteria, the EER helped to control utilization of substrate, so it has been successfully used to control glucose feeding in an industrial process.