The characteristics of Rhodobacter capsulatus ST410, a mutant of the wild strain B100 lacking hydrogenase activity, were investigated from the viewpoint of hydrogen production. When 30 mM DL-malate and 7 mM L-glutamate were used as carbon and nitrogen sources, respectively, in an argon atmosphere, a specific hydrogen evolution rate of 0.14 ml/h/mg-dry cells was obtained at 6600 Ix and 33 degrees C. The evolution rate strongly depended on the light intensity : the higher the light intensity, the larger the evolution rate became up to at least 6600 hi. R. capsulatus ST410 converted 60 mM malate to hydrogen at a yield of 68%, calculated as a percentage of the stoichiometric maximum for the complete conversion of the carbon source to Hz and CO2. On the other hand, when the wild strain was used under the same conditions, the yield was only 25%. R. capsulatus ST410 converted not only malate but also glucose and cellobiose to hydrogen with good yields (60% for 30 mM glucose and 66% for 7.5 mM cellobiose). Ethanolamine was found to be a good nitrogen source, which permitted a large amount of hydrogen to be evolved and also depressed the cell growth to low levels.