Algal starch photosynthetically accumulated in Chlamydomonas sp. MGA161 was metabolized to low-molecular-weight compounds - H-2, ethanol, glycerol, and acetate-under an O-2-free CO2 atmosphere. When cells in the logarithmic growth phase was fermented under CO2-anaerobic conditions, starch degradation in the alga was enhanced to 1.4-fold that in N-2-anaerobic fermentation. Due to the inhibition of hydrogenase under CO2, H-2 and acetate decreased, while ethanol and glycerol increased, when the atmosphere was changed from N-2 to CO2. Addition of CO, an inhibitor of hydrogenase, also stimulated starch degradation and changed the excretion pattern of the fermentation products.