a b s t r a c t In this paper, mitochondrial mutants of Chlamydomonas reinhardtii defective for respiratory complex I (NADH:ubiquinone oxidoreductase), complex III (ubiquinol cytochrome c oxido-reductase) and both complexes I and III were analyzed for H 2 photoproduction. Several parameters were followed during the S-deficiency stage and the anaerobic stage leading to H 2 photoproduction. At the early aerobic S-deficiency stage, starch and neutral lipids accumu-lated in all strains but their amount was significantly decreased in mutants compared to wild type. During the H 2 photoproduction process, whereas starch content strongly decreased in all strains, neutral lipid amount remained nearly unchanged, suggesting that starch degraded by glycolysis is the preferential substrate for energy production during anaerobi-osis. The mutants displayed a decrease in H 2 photoproduction correlating to the number of active mitochondrial proton-pumping sites lost in the strains. Our results thus highlight the critical role of oxidative phosphorylation during the first (aerobic) stage of S-starvation when carbon resources are accumulated.