Using sodium-dithionite as an oxygen scavenger, the influences of different light intensities and periods of anaerobic pre-incubation in the dark on H2-photoproductivity were studied with the green algaChlorella fusca. By measuring hydrogen production in the light using manometric and gas chromatographic methods the effectiveness of sodium dithionite in stabilizing photoproduction was established. For high rates of H2-photoproduction high light intensities up to 30,000 lux (580 W m-2) were necessary; these are comparable to those required for light saturation of oxygen photoproduction by this alga. AlthoughChlorella fusca produces H2 immediately after transition to anaerobic conditions, the optimum rate of H2 production was reached after a 5 h dark adaptation period only. The results obtained are discussed with respect to characteristics of H2-photoproduction by green algae: the initial burst kinetics, the light saturation, and the obligate period of anaerobic adaptation. It is concluded that H2-photoproduction byChlorella is an anaerobic photosynthetic process which occurs in the absence of CO2 and can be experimentally stabilized by exogenous oxygen scavengers.