Enterobacter aerogenes is a facultative anaerobic bacterium. It grows under both aerobic and anaerobic conditions. In our experiments, it assimilated glucose at a rate as high as 17 mmol glucose/(g-dry cell h) and consequently evolved hydrogen at near-equal speed. The yields of hydrogen, though, were small, at 1.0 mol from 1 mol glucose and 2.5 mol from 1 mol sucrose. The optimum temperature for bacterium growth was about 40 °C and the optimum pH for hydrogen evolution was roughly 5.7, though 7.0 was the optimum pH for growth. This bacterium immediately restored the respiratory function to its aerobic condition when cultivation was changed from anaerobic to aerobic conditions. Based on the mechanism of hydrogen evolution through a NADH (Nicotinamide Adenine Dinucleotide, reduced form) pathway, two methods were proposed to improve the yield of hydrogen from glucose. One was to inhibit the electron transport chain at the sites of the NADH dehydrogenase complex or cytochrome b-c1 complex and the other was to cultivate in the range of pH 5 to 4 under anaerobic conditions.