Unsynchronized Chlamydomonas reinhardtii cells subsequently deprived of sulfur produce H-2 under continuous illumination in the laboratory for 3-4 days. However, cultures grown outdoors will be exposed to day-and-night cycles that may synchronize their growth and cell division. While it is clear that only insignificant amounts of H-2 can be produced by sulfur-deprived cells during the night period, little work has been done to examine the effects of the light/dark cycles preceding sulfur deprivation on subsequent H-2 photoproduction. We show that (a) C reinhardtii cells exhibit synchronized growth and cell division in the presence of acetate, (b) cells with the highest specific rates of H-2 photoproduction also have the highest rates of biomass accumulation, and (c) the highest rates of starch and protein degradation coincide with the highest rates of formate and acetate accumulation, but not with H-2 photoproduction. This work shows that it is possible to maximize the production of H-2 by sulfur-depriving synchronized cultures at about 4 h after the beginning of the light period. (C) 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.