Glucose-limited cultures of Klebsiella aerogenes NCTC 418 (and the supposedly identical strain NCIB 418) possessed a glucose phosphoenolpyruvate (PEP) phosphotransferase activity that varied markedly and progressively with growth rate, from more than 250 nmol min-1 (mg dry wt cells)-1 at D = 0.1 h-1 to less than 100 nmol min-1 (mg dry wt cells)-1 at D = 0.8 h-1. When relieved of the glucose limitation, substrate was used at a rate that bore no precise relationship to the cells' phosphotransferase activity. Similarly, glucose-sufficient (phosphate- or potassium-limited) cultures metabolized glucose at high rates, whereas the cells possessed only moderate glucose PEP phosphotransferase activities. These results are compared with those reported for glucose-limited cultures of Escherichia coli and for variously limited cultures of K. aerogenes. Glucose-sufficient cultures, as well as glucose-limited cultures that had been temporarily relieved of glucose limitation, excreted partially oxidized products of glucose catabolism in considerable amounts. The relevance of this 'overflow' metabolism to studies of glucose transport using [U-14C]glucose is emphasized.