Bacillus subtilis alternately swims smoothly and tumbles; when administered repellent it only tumbles, but later resumes normal swimming and tumbling. Repellents of B. subtilis include membrane-active agents like uncouplers of oxidative phosphorylation and local anaesthetics and have previously been found to act in a fundamentally different way compared with attractants. It has been suggested previously that uncouplers act as repellents as a result of their ability to depolarize the membrane and that depolarization might effect flagellar function by causing a flux of Ca2+ into the cell. However, we found that there is no correlation between membrane depolarization and chemotaxis and no detectable flux of Ca2+ following tactic stimulation by uncouplers. Experiments with analogues of the uncoupler pentachlorophenol, all of which are weaker acids than pentachlorophenol, indicated that the anionic form of the uncoupler is the potent form and we propose that it binds to a certain membrane protein to cause release into the cytoplasm of the substance (ion, metabolite or protein) that controls tumbling frequency. Adaptation is assumed to occur when this excess is removed by active transport or metabolism.