The green alga, Chlamydomonas reinhardtii, is capable of sustained H 2 photoproduction when grown under sulfur-deprived conditions. This phenomenon is a result of the partial deactivation of photosynthetic O 2evolution activity in response to sulfur deprivation. At these reduced rates of water-oxidation, oxidative respiration under continuous illumination can establish an anaerobic environment in the culture. After 10±15 hours of anaerobiosis, sulfur-deprived algal cells induce a reversible hydrogenase and start to evolve H 2 gas in the light. Using a computer-monitored photobioreactor system, we investigated the behavior of sulfur-deprived algae and found that: (1) the cultures transition through ®ve consecutive phases: an aerobic phase, an O 2 -consumption phase, an anaerobic phase, a H 2 -production phase and a termination phase; (2) synchronization of cell division during pre-growth with 14:10 h light:dark cycles leads to earlier establishment of anaerobiosis in the cultures and to earlier onset of the H 2 -production phase; (3) re-addition of small quantities of sulfate (12.5±50 lM MgSO 4 , ®nal concentration) to either synchronized or unsynchronized cell suspensions results in an initial increase in culture density, a higher initial speci®c rate of H 2 production, an increase in the length of the H 2 -production phase, and an increase in the total amount of H 2 produced; and (4) increases in the culture optical density in the presence of 50 lM sulfate result in a decrease in the initial speci®c rates of H 2 production and in an earlier start of the H 2 -production phase with unsynchronized cells. We suggest that the effects of sulfur re-addition on H 2 production, up to an optimal concentration, are due to an increase in the residual water-oxidation activity of the algal cells. We also demonstrate that, in principle, cells synchronized by growth under light:dark cycles can be used in an outdoor H 2production system without loss of ef®ciency compared to cultures that up until now have been pre-grown under continuous light conditions.