Wild-type strains of Escherichia coli K-12 accumulate toxic concentrations of methylglyoxal when grown in medium containing adenosine 3',5'-monophosphate and either d-xylose, l-arabinose, or d-glucose-6-phosphate, independent of the presence of other carbon sources. Mutations at a locus called cxm specifically block methylglyoxal formation from xylose in the presence of adenosine 3',5'-monophosphate. Accumulation in medium containing xylose, studied in some detail, is dependent on the ability to utilize xylose and is associated with an increased rate of xylose utilization without changes in levels of xylose isomerase. These results suggest that adenosine 3',5'-monophosphate results in induction of excessively high levels of an early rate-limiting step in xylose metabolism. This step may be the transport of xylose into the cell. The resulting excessive rates of xylose catabolism could stimulate methylglyoxal formation by overburdening late steps in glycolysis.