Bacteriorhodopsin (BR), the light-driven proton pump of Halobacterium salinarium purple membrane, was produced in functional form as a two-chain protein by simultaneous expression in the fission yeast Schizosaccharomyces pombe of two separate structural genes, one coding for an aminoterminal BR fragment encompassing the first two transmembrane helices of BR, the other coding for the remainder of the protein. The fragments assemble spontaneously in vivo to yield functional BR which can be purified by immobilized metal ion affinity chromatography.