The effects of diquat, in the light, on chlorophyll bleaching and the fine structure of the cells of O. vUlgaris (211-11h) have been studied. Diquat causes bleaching of chlorophyll when the cells are illuminated in water or phosphate or bicarbonate buffer. Using cultures of uniform cell size it is shown that diquat causes considerable damage to cell membranes in less than 10 hr of illumination, but that inhibition of both photosynthesis and respiration precede any damage to the plastids visible under the electron microscope.