The effect of carbon dioxide on propionate formation during lactate and pyruvate metabolism by intact cells and cell-free preparations of Veillonella parvula M4 was examined. While the overall rate of lactate-U-14C degradation by intact cells was not altered, concentrations of unlabeled CO2 (to 50%) stimulated the evolution of 14CO2, and increased the absolute quantity of the propionate formed, while decreasing its specific activity. Acetate production from the labeled lactate, however, was not affected. Furthermore, propionate formation was not dependent on the presence of CO2 in the gas phase. The principle product of pyruvate-3-14C metabolism by extracts of the organism was acetate, although minor amounts of labeled propionate were also formed. Small quantities of oxaloacetate were detected during this metabolism by direct enzymatic assay and by separation of its hydrazone by thin-layer chromatography. Dialyzed crude extracts of the organism, treated with protamine sulfate and charcoal, were shown to catalyze the formation of oxaloacetate from pyruvate and CO2 in the presence of ATP when assayed both enzymatically with malic dehydrogenase and NADH, and with pyruvate-3-14C. This CO2-fixing activity could be attributed to pyruvate carboxylase, since the activity was Mg2+-dependent, was inhibited by avidin and aspartate, and was stimulated by biotin and CoA. Furthermore, the same extract preparation was capable of converting pyruvate-3-14C and CO2 to malate in the presence of NADH and NADPH indicating the presence of malic enzyme in the organism; this activity was confirmed by spectrophotometric means. As is the case with V. alcalescens, dialyzed extracts of V. parvula M4 catalyzed the exchange reaction between the carboxyl group of pyruvate and CO2; this exchange activity was stimulated by ATP.