When Porphyridium cruentum cells were illuminated with high fluence rate between 1900 and 4800 ~tmol photons m-2 s-1, a decrease in the photosynthetic activity of the cells was observed. Within the time frame of 20 rain, and under the fluence rates studied, the sum of photons to be absorbed by cells (mg of chlorophyll (Chl), sufficient to initiate photoinhibition was calculated to be 9235.8 gmol. The minimal specific light absorption rate to initiate photoinhibition in P. cruentum ranges between 2.29 and 4.26 gmol photons s-1 nag-1 chl.a. There was a linear relationship between the specific rate of photoinhibition and the specific light absorption rate. A photon number of 2.56 x 104 gmol mg -1 chl.a photoinhibited photosynthesis instantaneously. At 15~ no photoinhibitory effect was observed at 2300 gmol photons m-2 s-1 even after 45 rain of illumination. At the other extreme of 35~ 84% inhibition of photosynthetic activity was observed within 10 min of exposure to 2300 gmol photons m-2 s-~. Between 20 and 30 ~ C, the photoinhibitory effect was comparable. Photoinhibited P. cruentum cells recovered readily when transferred to low light (90 gmol photons m -2 s-1) and darkness, and the specific rate of recovery was independent of the light intensity to which the cells were exposed, during the photoinhibitory treatment.