The bacterial phosphoenolpyruvate:sugar phosphotransferase system (PTS) can repress the synthesis of certain catabolic enzyme systems in Salmonelly typhimurium and Escherichia coli. The present studies are concerned with an explanation of PTS-mediated repression by studying induction of the catabolic systems required for utilization of the non-PTS sugars glycerol, maltose, melibiose, and lactose. Repression of synthesis of these enzyme systems by various concentrations of PTS sugars was studied in wild type cells, in pts mutants, and in pts crr double mutants described in the accompanying reports (Saier, M. H., Jr., Simoni, R. D., and Roseman, S (1976) J. Biol. Chem. 251, 6584-6597: Saier, M. H., Jr., and Roseman, S. (1976) J. Biol. Chem., 6598-6605). The extent of repression was found to correlate with the degree of inhibition of uptake of the inducing non-PTS sugars. In both uninduced cells and cells fully induced for the respective transport system listed above, PTS sugars inhibited these transport systems. With both induced and uninduced cell types pts mutants were much more sensitive than wild type cells, while pts crr double mutants were completely resistant to the effects of the PTS sugars. Kinetic studies with the fully induced cells indicated that inhibition of transport by PTS sugars was reversible, that it affected the Vmax rather than the Km of entry, and that PTS sugars did not inhibit the efflux of the non-PTS sugars. These and other results indicate that inhibition did not result from competition between the PTS and non-PTS sugars for the transport systems, nor to competition for metabolic energy. Inhibition appears to require interaction of the PTS sugar with its membrane-bound Enzyme II complex, but whether concomitant uptake and phosphorylation of trace quantities of the PTS sugars is also required remains to be determined.