<jats:p> When an <jats:italic>Escherichia coli</jats:italic> mutant lacking the enzyme <jats:italic>N</jats:italic> -acetyl-glucosamine-6-phosphate (AcGN6P) deacetylase is grown in a succinate-mineral salts medium and exposed to an exogenous source of <jats:italic>N</jats:italic> -acetylglucosamine, approximately 20 to 30 pmoles of AcGN6P per μg of cell dry weight will accumulate in these cells. This accumulation occurs within 2 to 4 min after the addition of <jats:italic>N</jats:italic> -acetylglucosamine and is coincident with the production of a severe permanent catabolite repression of β-galactosidase synthesis. This repression does not occur if adenosine 3′,5′-cyclic phosphate (cyclic AMP) is added to the cells before AcGN6P accumulates. An immediate derepression occurs when cyclic AMP is added to cells that have already accumulated a large AcGN6P pool. These findings are consistent with the view that low-molecular-weight carbohydrate metabolites and cyclic AMP play key roles in the catabolite repression phenomenon, and that metabolites such as AcGN6P may participate in the represion mechanism by influencing either the formation or degradation of cyclic AMP in <jats:italic>E. coli</jats:italic> .