The photosynthetic reaction center-light harvesting 1 complex from Rhodopseudomonas viridis was purified and reconstituted into two-dimensional crystals. The single-layered crystalline sheets with lattice parameters a=b=133.3 A and gamma=120 degrees were investigated by electron cryo-microscopy and the projection map at 10 A resolution was calculated. The opening diameter of the light-harvesting ring of 72 A is sufficient to allow slight movement of the reaction center within the ring. Based on characteristic features observed in the projection map, the mechanism of energy transfer from the light-harvesting 1 complex to the reaction center was discussed.