2-Keto-3-deoxy-gluconate (KDG), an intermediate of the hexuronate pathway in Escherichia coli K-12, is utilized as the sole carbon source only in strains derepressed for the specific KDG-uptake system. KDG is metabolized to pyruvate and glyceraldehyde-3-phosphate via the inducible enzymes KDG-kinase and 2-keto-3-deoxy-6-phosphate-gluconate (KDPG) aldolase. However, another inducible pathway, where the KDG is the branch point, has been demonstrated. Genetic studies of the KDG degradative pathway reported in this paper led to the location of KDG kinase-negative and pleiotropic constitutive mutations. The kdgK locus, presumably the structural gene of the kinase, occurs at min 69 and is co-transducible with xyl. The mutants, simultaneously constitutive for the uptake, kinase, and aldolase, define a kdgR locus at min 36 between the co-transducible markers kdgA and oldD. As to the nature of the control exerted by the kdgR product, we have shown the following. (i) Thermosensitive mutants of the kdgR locus are inducible at low temperature but derepressed at 42 C for the three operons-kdgT (transport system), kdgK, and kdgA (KDPG aldolase). (ii) The kdgR(+) allele is dominant to the kdgR constitutive allele. (iii) A deletion in kdgA extending into the regulatory gene, kdgR, leads to a constitutive expression of the nondeleted operons-kdgT and kdgK. These properties demonstrate that the kdg regulon is negatively controlled by the kdgR product. It is presumed that differences in operator and in promotor structures could explain the strong decoordination, respectively, in the induction and catabolic repression, of these three enzymes activities.