A marine cyanobacterium, Anabaena sp. TU37-1, exhibited stable production of hydrogen and oxygen in closed vessels. About 8.4 and 4.3 mL (at atmospheric pressure) of hydrogen and oxygen accumulated, respectively, in flasks with 20 mL gas phase during 48 h incubation. Thus, concentration of H-2 and O-2 became 26 and 13% of the gas phase, respectively. Duration of hydrogen production was prolonged by the periodic gas replacement in the reaction vessel. The conversion efficiencies of photosynthetically active radiation (fluorescent light, 22 W/m(2)) to hydrogen were 2.4 and 2.2% during the initial 12- and 24-h incubation periods, respectively.