S U M M A R YActinomyces naeslundii fermented glucose primarily by the EmbdenMeyerhof pathway, as based on 14C-glucose fermentation data and enzyme studies. Enzymes of the oxidative pentose phosphate cycle were also present, but functioned only to a minor extent. Growth on glucose was increased 2-to 4-fold in the presence of substrate amounts of CO, or 02. This increase was attributed to the additional energy (ATP) made available from the breakdown of pyruvate to acetyl coenzyme A. In the absence of CO, or O,, pyruvate was reduced to lactate. The weight of organism produced/ mole ATP ( YATP) was 15-1 8 g. units under anaerobic conditions with C02, dependent on the growth medium, and 20 under aerobic growth conditions.