An Escherichia coli mutant using an NAD-linked dehydrogenase instead of an ATP-dependent kinase as the first enzyme for glycerol dissimilation excreted dihydroxyacetone during the initial phase of growth. The intermediate was salvaged as growth of the culture advanced. The transient loss of the intermediate into the medium appeared to be partly determined by variation of the level of glycerol dehydrogenase with growth conditions. With up to 2% casein hydrolysate as the carbon and energy source, the cellular level of the dehydrogenase increased 1 order of magnitude at the end of growth. This increase was probably caused by the depletion of certain metabolites and was prevented by the addition of pyruvate or glucose to the growth medium. The repressive effect of these compounds was not lifted by the addition of cyclic AMP. Diminution of oxygen tension in the culture medium with increased cell density was not directly responsible for the increase of the enzyme level. Thus, neither catabolite repression nor respiratory repression was implicated as an important control mechanism in the synthesis of this enzyme. Since increases in the specific activity of the enzyme in cell extracts reflected increases in the concentration of the enzyme protein, post-translational control was also not involved. A novel kind of regulation of gene expression is indicated.