Halobacteria are extremely specialized organisms. They live exclusively in saturated solutions of common salt. The cell membrane of these bacteria exhibit insular regions which can be isolated by membrane fractionation. These regions consist of a lipid matrix containing bateriorhodopsin molecules in a hexagonal crystalline arrangement. Bacteriorhodopsin is a deep purple retinal-protein complex (“purple membrane”). The purple membrane functions as a light energy converter. --How can such a differentiated membrane region arise? In uiuo studies on the biosynthesis of the purple membrane showed another cell membrane fraction, the so-called brown membrane, to be a biosynthetic precursor. Bacterioopsin (the retinal-free protein) is initially incorporated into the brown membrane and can only form the purple membrane by crystallization in an energy-dependent reaction after prior reaction with retinal. This reaction is reversible. Removal of the retinal by formation of retinal oxime causes the purple membrane regions to disappear. Reconstitution of the bacteriorhodopsin by addition of retinal regenerates the purple membrane.