With a glucose-limited culture of Klebsiella aerogenes, growing at a fixed dilution rate (0.4 h-1), the specific respiration rate varied progressively as a function of the transmembrane K+ gradient. The latter was varied by changing the input K+ concentration and, under these conditions, the specific respiration rate was linearly related to the electrochemical potential of the K+ gradient. Increasing or decreasing the transmembrane K+ gradient in putatively potassium-limited cultures elicited marked changes in respiration rate consistent with the conclusion that the exceptionally high respiration rates expressed by fully glucose-sufficient potassium-limited cultures (i.e., values in excess of 25 mmol O2/g dry weight organisms . h, at D = 0.4 h-1) are necessary to scavenge traces of K+ from the environment and hence maintain an exceptionally high transmembrane K+ gradient.