THE sole protein found in the purple membrane of Halobacterium halobium contains retinal covalently bound by means of a protonated Schiff base linkage to lysine1,2. The only established function of this protein is to act as a light-driven proton pump producing a transmembrane proton gradient, which is coupled to ATP synthesis in the living organism2,3. In so far as the retinal prostheticgr oup resembles that found in the visual pigment of vertebrates, rhodopsin, the name bacteriorhodopsin has been widely adopted for this protein3. Considerable work has been done on the biochemistry and photochemistry of the purple membrane, both for its intrinsic interest as a unique photosynthetic membrane and with respect to the identification of possible mechanistic relationships between its photochemical cycle and that of the visual process. We report here a study of the later stages of the photochemical cycle by flash photometry with plane polarised light. Our results indicate conformational change in the bacteriorhodopsin molecule at the site of the visible chromophore.