Lactaldehyde dehydrogenase (E.C. 22.214.171.124) of Escherichia coli has been purified to homogeneity. It has four apparently equal subunits (molecular weight 55,000 each) and four NAD binding sites per molecule of native enzyme. The enzyme is inducible, only under aerobic conditions, by at least three different types of molecules, the sugars fucose and rhamnose, the diol ethylene glycol and the amino acid glutamate. The enzyme catalyzes the irreversible oxidation of several aldehydes with a Km in the micromolar range for alpha-hydroxyaldehydes (lactaldehyde, glyceraldehyde, or glycolaldehyde) and a higher Km, in the millimolar range, for the alpha-ketoaldehyde methylglyoxal. It displays substrate inhibition with all these substrates. NAD is the preferential cofactor. The functional and structural features of the enzyme indicate that it is not an isozyme of other E. coli aldehyde dehydrogenases such as glyceraldehyde phosphate dehydrogenase, glycolaldehyde dehydrogenase, or acetaldehyde dehydrogenase. The enzyme, previously described as specific for lactaldehyde, is thus identified as a dehydrogenase with a fairly general role in aldehyde oxidation, and it is probably involved in several metabolic pathways.