A membrane protein complex, photosynthetic reaction center purified from the thermophilic purple sulfur bacterium, Chromatium tepidum has been crystallized from a phosphate buffer containing a detergent, n-octyl-beta-D-glucopyranoside and a precipitant, polyethylene glycol 4000. The crystals diffracted X-rays beyond 3A resolution with synchrotron radiation and are suitable for high-resolution X-ray crystallographic studies. The crystals belong to the orthorhombic space group P2(1)2(1)2(1) with unit-cell dimensions of a = 136A, b = 197A, and c = 82A. Assuming that they contain one reaction center complex in the asymmetric unit, VM was calculated to be 4.3 A3/Da, which agrees with the values obtained in the membrane protein complexes.