The thermophilic glycolytic anaerobe Thermoanaerobacter wiegelii grows over the pH range 5.1-7.7, and no growth is observed below pH 5.0 or above 7.7. When T. wiegelii was grown in pH-uncontrolled batch culture, glucose was fermented to ethanol, acetate, and lactate. Small amounts of lactic acid were produced once the external pH reached 6.0, and a fructose-1.6-diphosphate (FDP) activated lactate dehydrogenase (LDH) was detected in cell-free crude extracts. Maximal activation of LDH by FDP was observed at pH 6.2. As the pH of the medium declined from 6.7 to 5.1 due to the production of acetate and lactate, the total protonmotive force (deltap) remained between - 110 and - 130mV, and the membrane potential (dekltapsi) decreased from -104 to -65mV. This decrease in deltapsi was paralleled by an increase in the chemical gradient of protons (ZdeltapH) from -31 to -62mV at pH 5.1. Based on these results, T. wiegelii maintained a small deltapH (0.3-0.9 units, inside alkaline) as the medium pH declined and interconverted deltapsi to ZdeltapH to maintain the total deltap relatively constant. Intracellular potassium decreased from 150 mM at pH 6.70 to 50 mM at pH 5.1, and this represented a 33-mV decline in the transmembrane chemical potential of potassium. The ability to synthesize ATP remained constant as the external pH declined, and therefore metabolic energy per se was not a critical aspect of pH sensitivity.