1. The time course of aerobic photobleaching of the chlorophyll 6-free alga Vischeria stellata is of a biphasic nature. Initially the bleaching rate is about 10 times higher than under prolonged exposure to high light intensities. 2. Centrifugation considerably enhances the aerobic photobleaching rate of chloroplast fragments. Concomittantly the 685 nm component of the low-temperature fluorescence spectrum is markedly increased. The relationship between the intensity of this component and the bleaching capacity resembles an exponential one. 3. As the photobleaching proceeds, the red absorption maximum shifts to longer wavelengths. This relationship is linear. 4. Both photosystems are involved in the initial bleaching. 5. The results are discussed in terms of the spill-over model for excitation energy in photosynthesis.