During batch growth of Alcaligenes eutrophus on benzoate-acetate mixtures, benzoate was the preferred substrate, with acetate consumption being delayed until the rate of benzoate consumption had diminished. This effect was attributed to a transcriptional control of the synthesis of acetyl coenzyme A (acetyl-CoA) synthetase, an enzyme necessary for the entry of acetate into the central metabolic pathways, rather than to a biochemical modulation of the activity of this enzyme. Analysis of a 2.4-kb mRNA transcript hybridizing with the A. eutrophus acoE gene confirmed this repression effect. In a benzoate-limited chemostat culture, derepression was observed, with no increase in the level of expression following an acetate pulse. Benzoate itself was not the signal triggering the repression of acetyl-CoA synthetase. This role was played by catechol, which transiently accumulated in the medium when high specific rates of benzoate consumption were reached. The lack of rapid inactivation of the functional acetyl-CoA synthetase after synthesis has been stopped enables A. eutrophus to retain the capacity to metabolize acetate for prolonged periods while conserving minimal protein expenditure.