The purple membrane of Halobacterium halobium contains only one protein, bacteriorhodopsin, which closely resembles the visual pigments of animals. Light flashes cause a rapid transient shift of its absorption maximum from 560 to 415 nm. This shift is accompanied by release and uptake of protons. Respiring cells acidify the medium in the dark; if they contain purple membrane their O(2) consumption is reduced in the light. Starved or anaerobic cells containing purple membrane, in the absence of any apparent source of energy, generate and maintain a proton gradient across the cell membrane as long as they are exposed to light. We postulate that the light-generated proton gradient arises from a vectorial release and uptake of protons by bacteriorhodopsin, which is suitably oriented in the cell membrane and under continuous illumination oscillates rapidly between the long- and short-wavelength form. Preliminary results indicate that the gradient in H. halobium plays the central role in energy coupling attributed to such electrochemical gradients by Mitchell's chemiosmotic theory.