We have examined characteristics of the photoinhibition of photosynthesis which occur in the unicellular cyanobacterium Microcystis aeruginosa, following exposure to photon fluence rates in excess of those required for growth. Photoinhibition occurs following exposure of cells to a photon fluence rate of 1,000 μmol m(-2) s(-1), which is manifested as a decrease in either light-limited CO2 fixation or light-saturated CO2-dependent O2 evolution. The extent and rapidity of this photoinhibition is greatly enhanced under CO2-depleted conditions. Experiments in which cultures were sparged with different gases indicate that photoinhibition is not an obvious consequence of elevated O2 tensions, unlike the photooxidative bleaching of photosynthetic pigments. Comparative studies on the photoinactivation of CO2-dependent O2 evolution and of the methyl viologen-dependent Mehler reaction, in whole cells, indicate that a primary site of light damage is within the photosynthetic electron-transport reactions and that carbon fixation is initially unaffected.