In Escherichia coli, an aldehyde dehydrogenase that catalyzes the oxidation of L-lactaldehyde to L-lactate is induced not only by L-fucose, L-rhamnose or D-arabinose, but also by growth in the presence of glutamate or amino acids yielding glutamate, with the exception of proline. Induction by these amino acids requires glutamate accumulation. 4-Aminobutyric acid also induces this aldehyde dehydrogenase through its transamination to glutamate. Growth on 2-oxoglutarate, the tricarboxylic acid cycle intermediate with which glutamate is in equilibrium, also induces this aldehyde dehydrogenase. Conditions in which the conversion of 2-oxoglutarate into glutamate is highly restricted displayed unchanged rates of induction by 2-oxoglutarate, indicating that glutamate induces the aldehyde dehydrogenase through 2-oxoglutarate formation. Evidence is presented showing that Lfucose-and 2-oxoglutarate-inducing systems share the same regulatory protein. Induction by growth on either of these two compounds is repressed both by glucose and by glycerol. Addition of CAMP to these cultures partially recovers the glucose-repressed aldehyde dehydrogenase activity, while this nucleotide has no effect on the glycerolmediated repression. These results indicate that ald is under carbon regulation mediated by at least two different mechanisms. Growth of Escherichia coli on L-fucose [l], L-rhamnose [2]or L-1 ,2-propanediol [3] induces an aldehyde dehydrogenase enzyme which we refer to as ALDH. This enzyme is also known as lactaldehyde dehydrogenase because it oxidizes Llactaldehyde, a common intermediate in the utilization of these carbon sources. In another context, studying the utilization of ethylene glycol by E. coli mutants, Boronat et al. [4] described the involvement of a glycolaldehyde dehydrogenase activity, which was identified as a function of the same ALDH enzyme [S]. In this metabolic pathway, the enzyme oxidizes glycolaldehyde to glycolate. The formation of glycolaldehyde and its conversion to glycolate has also been reported in the aerobic metabolism of D-arabinose [6].Lastly, Baldoma and Aguilar [2] have described that ALDH is induced in wild-type E. coli by the addition of glutamate to late-exponential-phase cultures grown on glycerol, although in this ca \e inducer molecule and the mechanism in Iolved are not known. Veither succin+ semialdehyde nor glulamic 4-semialdehyde, both intermediare aldehydes in pathways derived from glutamate, was found to be inducer or substrate of the dehydrogenase [2, 71.ALDH has been purified and the homogeneous enzyme has k e n shown to present a broad substrate specificity,