X-ray analysis of three-dimensional crystals of the photosynthetic reaction center from the purple bacterium Rhodopseudomonas viridis led to an electron density distribution at 3 A resolution calculated with phases from multiple isomorphous replacement. The protein subunits of the complex were identified. An atomic model of the prosthetic groups of the reaction center complex (4 bacteriochlorophyll b, 2 bacteriopheophytin b. 1 non-heme iron, 1 menaquinone, 4 heme groups) was built. The arrangement of the ring systems of the bacteriochlorophyll b and bacteriopheophytin b molecules shows a local 2-fold rotation symmetry; two bacteriochlorophyll b form a closely associated, non-covalently linked dimer ("special pair"). A different local 2-fold symmetry axis is observed for the heme groups of the cytochrome part.