Thiobacillus thiooxidans was grown at constant pH. No growth was obtained above pH 4.3. Sulfur was oxidized over the entire range tested (pH 0.9 to 7.0). This was confirmed by pH shift experiments. Carbon dioxide fixation stopped 30 min after shift from low pH to neutral values. On return to high acidity, the rate of CO2 assimilation increased but not always to the original value. The intracellular binding of glycerol which is passively permeable was also inhibited by raising the pH of the medium. The data suggested that ATP formation may be inhibited at neutral pH. Intracellular ATP pool was reduced by 80% on neutralization of the acid growth medium. These results are compared to recent studies on acid-base transients in T. novellus by Cole and Aleem (1971).