Effects of pH on the glucose and lactate metabolism by the washed cells of Actinomyces naeslundii genospecies 1 and 2 under anaerobic and aerobic conditions were studied. The rate of acid production from glucose was the highest at pH 7.0 and decreased as the pH lowered to 4.5, irrespective of atmospheric conditions. The anaerobic end-product in the absence of bicarbonate was mainly lactate, while in the presence of bicarbonate the rate of acid production increased 1.8-2.5 times with the production of formate, acetate and succinate in addition to lactate. Under aerobic conditions, the cells produced acids from glucose along with oxygen consumption and the end-product was mainly acetate. In contrast to the glucose metabilism, the cells produced base from lactate along with oxygen consumption. The rates of base production and oxygen consumption were the highest at pH 5.5. The end-products from lactate were acetate and pyruvate. These results indicate that oral actinomyces has a various activity of glucose and lactate metabolism at a wide range of environmental pH and suggest its flexibility in surviving in dental plaque, where the environmental factors fluctuate.