Extrusion of sodium ion from cells of Escherichia coli was measured using an Na⁺ electrode. When oxygen was supplied to an anaerobic cell suspension, extrusion of Na+ was observed. The addition of glucose under anaerobic conditions also caused Na⁺ efflux. The extrusion of Na⁺ energized by respiration and glycolysis was completely inhibited by a proton conductor, carbonyl cyanide m-chlorophenylhydrazone. These observations are consistent with the view that Na⁺ transport occurs secondarily to H⁺ circulation. Interestingly, induction of the melibiose transport system, which is coupled to Na⁺, greatly enhanced Na⁺ transport activity.