Actinomyces viscosus and Actinomyces naeslundii were grown in chemostat cultures with glucose or lactate as the sole sources of energy. Growth on lactate was dependent on oxygen. Lactate was oxidised partly to acetic acid. As judged from the low carbon recoveries in acidic degradation products, it is concluded that a substantial part of the lactate was oxidised to carbon dioxide. Assuming that 1 ATP is generated in the transfer of an electron pair to oxygen, YATP values of 10-11 were calculated from the molar growth yield on lactate and product formation. Although all cell 3 test strains oxidised lactate, one strain could not grow on lactate as the sole source of energy. The rate of oxygen uptake with lactate as substrate was higher for cells grown aerobically than of cells grown anaerobically, indicating that lactate oxidation was induced by oxygen. It is reasonable to assume that lactate oxidation is mediated either by lactate dehydrogenase or lactate oxidase, but the pathway in Actinomyces sp. is not yet known. Lactate consumption may be an important trait of Actinomyces sp. that live in an environment limited in energy sources for most of the time.