Evidence is presented that in Acinetobacter calcoaceticus oxidation of glucose to gluconate by the periplasmic quinoprotein glucose dehydrogenase (EC 1 . 1 .99.17) leads to energy conservation. Membrane vesicles prepared from cells grown in carbon-limited chemostat culture exhibited (1) a high rate of glucose-dependent oxygen consumption and gluconate production, (2) glucosemediated cytochrome reduction, (3) uncoupler sensitive, glucose-dependent generation of a membrane potential and (4) glucose-driven accumulation of amino acids. Furthermore, oxidation of glucose to gluconate by whole cells was associated with ATP synthesis. These results confirm and extend previous observations that periplasmic glucose oxidation can act as a driving force for energy-requiring processes. It is therefore concluded that the incomplete oxidation of glucose by bacteria may serve as an auxiliary energy-generating system.