Sorbitol-fermenting strains of Streptococcus sanguis and Streptococcus mitior were grown both anaerobically and in the presence of oxygen in a sorbitol-containing complex medium. Washed-cell suspensions were incubated with an excess of sorbitol, and the production of lactate, formate, ethanol, and acetate was analyzed. Moreover, we determined the lactate dehydrogenase and pyruvate formate-lyase activities in cell-free extracts of anaerobically grown cells. The anaerobically grown cells produced lactate, formate, ethanol, and acetate under anaerobic conditions. When these cells were exposed to air, the amounts of formate, ethanol, and acetate were reduced in comparison with those of the strictly anaerobic cells. Cells grown in the presence of oxygen only produced detectable levels of lactate and acetate. Anaerobically grown cells possessed lactate dehydrogenase and pyruvate formate-lyase activities under strictly anaerobic conditions. The level of pyruvate formate-lyase was dramatically reduced when cells were exposed to air, while the level of lactate dehydrogenase was about the same as that under strictly anaerobic conditions. Thus, the results indicate that S. sanguis and S. mitior both metabolize sorbitol differently under anaerobic and aerobic conditions. This difference may depend on the oxygen-sensitivity of the pyruvate formatelyase of these micro-organisms.