A photobioreactor has been designed and constructed for hydrogen production by Rhodobacter. sphaeroides O.U.001 (DSM 5864). Two phases have been distinguished upon illumination at argon atmosphere, the growth phase and the hyrogen production phase. Anaerobic growth was observed at a wide range of pH (6-9) and temperature (25-40 degrees C) only under illumination. Rhodobacter sphaeroides O.U.001 photoproduced hydrogen after a certain limiting cell concentration has been reached, however, high cell mass caused a decrease in hydrogen evolution rate. Therefore an optimum cell concentration range has been found (1.6-1.8 mg dry wt/ml). Hydrogen photoproduction rate was higher at 31 degrees C than at 36 degrees C. High purity hydrogen gas has been produced which was composed of 99 % of hydrogen and 1 % of carbondioxide on argon free basis.