The synthesis of alpha-ketoglutarate dehydrogenase by a species of Acinetobacter growing in the presence of C4 compounds (succinate or malate) and in the presence of a C2 compound (acetate), as sole carbon sources, has been investigated. The rate of synthesis of this enzyme is increased rapidly when cells are inoculated into a succinate medium, and growth is initiated essentially without a lag period. The enzyme is synthesized after some lag period in the presence of malate and growth begins as the rate of enzyme synthesis begins to increase. On the contrary, growth begins immediately upon inoculation of the cells into an acetate medium. After a few hours of growth the level of alpha-ketoglutarate dehydrogenase begins to fall and apparent repression of synthesis occurs. These results are discussed in the light of isocitrate lyase levels in the cells at the same time periods and the evidence indicates that when levels of alpha-ketoglutarate dehydrogenase are high, those of isocitrate lyase are low. This suggests a control mechanism regulating the concurrent operation of the tricarboxylic acid cycle and the glyoxylate by-pass. Data are presented also which correlate substrate oxidation by succinate-grown cells and the cellular levels of alpha-ketoglutarate dehydrogenase.