Measurements with a PAM fluorometer showed that the photochemical activity of photosystem II (PS II) in sulfur-deprived Chlamydomonas reinhardtii cells (media TAP-S) decreases slowly under aerobic conditions. In a closed cultivator, when the rate of O2 photosynthetic evolution declines below the rate of respiration, the cell culture is under anaerobic conditions in which the activation of hydrogenase and the production of hydrogen take place. We found that the slow decrease in PS II activity is followed by an abrupt inactivation of PS II centers just after the onset of anaerobiosis. This fast PS II inactivation is reversed by aeration of the media and is accompanied by an increase in the fluorescence parameter Ft. Moreover, the rate of the abrupt PS II inactivation diminished after the addition into the medium of electron acceptors such as CO2 (carbonate-bicarbonate buffer), NO3- and SO4(2-) , the assimilation of which in chloroplasts requires a lot of reductants. We suggest that the PS II inactivation is due to the overreduction of the plastoquinone pool after the onset of anaerobiosis.