In the photosynthetic bacterium Rhodobacter sphaeroides, a water-soluble cytochrome c₂ (cyt c₂) is the electron donor to the photosynthetic reaction center (RC), a membrane- bound pigment-protein complex that is the site of the primary light-induced electron transfer. To determine the interactions important for docking and electron transfer within the transiently-bound complex of the two proteins, the RC and cyt c₂ were co-crystallized and the structure determined by x-ray diffraction. Cyt c₂ reduces the photo-oxidized RC donor (D⁺), a bacteriochlorophyll dimer, in the co-crystals in 0.9 microsecond, which is the same time as measured in solution. This provides strong evidence that the structure of the complex in the region of electron transfer is the same in the crystal and in solution. X-ray diffraction data were collected using synchrotron radiation to a resolution of 2.40 Å, and the coordinates were refined to an R-factor of 22% (R_free = 26%). The intermolecular interactions important for binding and electron transfer will be described. The binding interface can be divided into two domains: (i). A short-range interaction domain that includes groups exhibiting non-polar interactions, hydrogen bonding, and a cation-pi interaction. This domain is improtant for electron transfer and contributes to the strength and specificity of cyt c₂ binding. (ii). A long-range, electrostatic interaction domain that contains solvated complementary charges on the RC and cyt c₂. This domain, in addition to contributing to the binding, may steer the unbound proteins toward the right conformation.