1. Changes in the fluorescence yield of aerobic Chlorella vulgaris have been measured in laser flashes of 15 ns, 30 ns and 350 ns half time. The kinetics after the first flash given after a 3 min dark period could be simulated on a computer using the hypothesis that the oxidized acceptor Q and primary donor P+ are fluorescence quenchers, and Q- is a weak quencher, and that the reduction time for P+ is 20-35 ns. 2. The P+ reduction time for at least an appreciable part of the reaction centers was found to be longer after the second and subsequent flashes. In the first 5 flashes an oscillation was observed. Under steady state conditions, with a pulse separation of 3 s, a reduction time for P+ of about 400 ns for all reaction centers gave the best correspondence between computed and experimental fluorescence kinetics.