A rapid temperature drop during anaerobic acidogenesis in an anaerobic reactor system resulted in the sharp suppression of carbohydrate decomposition and production of volatile fatty acids. When the temperature was dropped from 30 degrees C to 25, 20, 15 degrees C sequentially, the numbers of bacteria were slowly reduced without showing temperature shock. The acidogenesis, however, was dramatically affected after each temperature drop; the removal efficiency of carbohydrate was reduced from 92% to 84%, 72%, 25% with showing the minima of 78%, 52% and 10% due to the rapid temperature drop respectively. This indicates that the acidogens lose the activities momentarily during the rapid temperature drop and require certain period of time to recover the activities at the adjusted fermentation temperature.