The thermophilic bacterium, Moorella sp. HUC22-1, newly isolated from a mud sample, produced ethanol from H(2) and CO(2) during growth at 55 degrees C. In batch cultures in serum bottles, 1.5 mM ethanol was produced from 270 mM H(2) and 130 mM CO(2) after 156 h, whereas less than 1 mM ethanol was produced from 23 mM fructose after 33 h. Alcohol dehydrogenase and acetaldehyde dehydrogenase activities were higher in cells grown with H(2) and CO(2) than those grown with fructose. The NADH/NAD(+) and NADPH/NADP(+) ratios in cells grown with H(2) and CO(2) were also higher than those in cells grown with fructose. When the culture pH was controlled at 5 with H(2) and CO(2) in a fermenter, ethanol production was 3.7-fold higher than that in a pH-uncontrolled culture after 220 h.