The thermophilic bacterium, Moorella sp. HUC22-1, newly isolated from a mud sample, produced ethanol from H-2 and CO2 during growth at 55 degreesC. In batch cultures in serum bottles, 1.5 mm ethanol was produced from 270 mm H-2 and 130 mm CO2 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 CO2 than those grown with fructose. The NADH/NAD(+) and NADPH/NADP(+) ratios in cells grown with H-2 and CO2 were also higher than those in cells grown with fructose. When the culture pH was controlled at 5 with H2 and Co-2 in a fermenter, ethanol production was 3.7-fold higher than that in a pH-uncontrolled culture after 220 h.