Fermentative hydrogen evolution by Enterobacter aerogenes strain E.82005 was studied under controlled pH. The pH of culture was found to have significant influences on the hydrogen evolution and cell growth. The optimum pH for hydrogen evolution was around pH 6.0. The evolution rate of hydrogen was 520 ml/(l-culture・h) at pH 6.0 and 40.5°C. This rate seemed to be the fastest as compared with the other microorganisms. Mechanism of hydrogen evolution from excess NADH was discussed binding with the pH activity.