The stimulatory effect of acetyl-CoA and the inhibitory effect byL-aspartate and some intermediates of the tricarboxylic acid cycle in the assimilation of CO2by crude extracts of group D streptococci suggest that the pyruvate carboxylase ofStreptococcus faeciumand the phosphoenolpyruvate carboxylase ofS.bovisare allosteric enzymes. This implies that these enzymes are sites for the control of the amount of aspartate and of the tricarboxylic acid cycle intermediates synthesized.