Changes in the membrane potential, pH gradient, proton motive force, and intracellular pH of Escherichia coli were followed during the chemotactic responses to a variety of potentially membrane-active compounds. Lipophilic weak acids, decreases in extracellular pH, and nigericin each caused a repellent response. Lipophilic weak bases, increases in extracellular pH, and valinomycin in the presence of K+ each caused an attractant response. Changes in membrane potential, pH gradient, and proton motive force did not correlate with the behavioral responses to these treatments, but changes in intracellular pH did correlate. Furthermore, the strength of the response to a weak acid was correlated with the magnitude of the change of the intracellular pH, and many compounds which could alter the intracellular pH were found to be chemotactically active. Apparently these attractants and repellents are not detected by specific chemoreceptors but rather are detected via the ability of cells to sense and respond to changes in intracellular pH. The pathway of sensory transduction which proceeds through methyl-accepting chemotaxis protein I was found to be involved in the response to a change in intracellular pH.