Abstract : The objective of the research program was to characterize control mechanisms that exercised negative regulation on the expression of genes for aromatic catabolism in bacteria. The three sets of genes selected for investigation were ben, encoding enzymes that convert benzoate to catechol; cat, encoding enzymes tht convert catechol to citric acid cycle intermediates; and pca, encoding enzymes that convert protocatechuate to citric acid cycle intermediates. Our initial approach was to clone the structural genes from Acinetobacter calcoaceticus and Pseudomonas putida, bacteria in which aromatic catabolism has been well characterized, because we knew that regulatory genes frequently flanked the structural genes. Our efforts were largely successful, and we identified two cloned regulatory genes from P. putida. One of these, pcaR, exercises positive control over three unlinked gene clusters. The other, catR, exercises negative control over the tightly linked catBC genes. The latter gene is analogous in many respects to another regulatory gene, also designated catR, that we have cloned from A. calcoaceticus.