The B800-820 light-harvesting complex, an integral membrane protein, from Rhodopseudomonas acidophila strain 7750 has been crystallized. The tabular plates have a hexagonal unit cell of a = b = 121·8 A ̊ and c = 283·1 A ̊ and belong to the space group R32. X-ray diffraction data have been collected to 6 Å resolution, using an area detector on a rotating anode source. The B800-820 light-harvesting complex is comprised of four low molecular weight apoproteins (B800-820α1, B800-820α2, B800-820β1 and B800-820β2). Polyacrylamide gel electrophoresis shows that the complex exists as an oligomeric assembly, with an apparent molecular weight of 92,000. © 1992.