Glucose uptake by the cellulclytic rumen anaerobe, Bacteroides succinogenes S85, was measured under conditions that maintained anaerobiosis and osmotic stability. This organism was found to possess a highly specific, active transport mechanism for glucose. Evidence for a phosphoenol-pyruvate:g1ucose phosphotransferase system was not detected. Compounds that inhibit electron transport systems (non-heme iron chelators, and sulfhydryl reagents) were effective inhibitors of glucose uptake. The strongest inhibitors were compounds (proton and metal ionophores) that interfere with maintenance of the proton motive force. Compounds which interfere with ATP synthesis also inhibited glucose uptake, but a role for ATP in energizing uptake could not be inferred from these results. Oxygen prevented glucose uptake (75% inhibition), reflecting possible active sulfhydryl centers (above) or autooxidation of electron transport components. The results suggest the fumarate reductase-coupled electron transport system of B. succinogenes can generate a proton motive force that is used to energize glucose uptake. Na+ and Li+. but not K+, stimulated glucose uptake and may partly account for the growth requirement of B. succinogenes for Na+. However, the data were insufficient to conclude that glucose uptake occurs by a Na+ symport mechanism. Spheroplasts of B. succinogenes transported glucose as well as whole cells, indicating glucose uptake is not dependent on a periplasmic glucose binding protein. A variety of sugars including the nonmetabolizable analog, [inversely proportional symbol]-methylglucoside. did not inhibit glucose uptake. Only cellobiose and 2-deoxyglucose were active and neither behaved as a competitive inhibitor. Metabolism of both sugars was probably responsible for the inhibition. Cellobiose-grcwn B. succinogenes showed a reduced ability to transport glucose compared to glucose-grown cells. This may indicate regulation of synthesis of the glucose carrier protein by cellobiose through a mechanism other than catabolite repression. Differences in the ability to transport glucose were detected between transition cells (transition from lag to log phase of growth) and log-phase cells. However, the differences were not due to different glucose transport mechanisms. Alterations in the structural integrity of the cell envelope, as reflected by osmotic- and cold-sensitivity features of transition and log cells, may have affected the glucose uptake abilities in these cell types.