The structure of the major protein constituent of photosynthetic membranes in higher plants, the chlorophyll a/b-light harvesting complex (LHC), was studied by x-ray diffraction and electron microscopy. The LHC was purified from Triton X-100 solubilized thylakoid membranes of the pea, and contained 6 mol of chlorophylls a and b per mole of a polypeptide of 27,000 molecular weight. X-ray diffraction showed that in the presence of 10 mM MgCl2, purified LHC forms planar aggregates that stack with a period of 51 A. Within each layer, LHC molecules pack with a center-to-center distance of 85 A but without long-range order. However, when LHC is incorporated into single-walled vesicles of plant lecithin, the addition of NaCl above 10 mM, or MgCl2 above 2 mM, led to the formation of plaques of hexagonal lattices, with a lattice constant of 125 A. The large domain size and high degree of order in the plane of the membrane are evident from the sharp lattice lines observed to 7 A resolution on the equator of the x-ray pattern. Freeze-fracture electron micrographs demonstrated an aligned stacking of the lattices in adjacent membranes, resulting in crystallinity in the third dimension over short distances. Micrographs of negatively stained membranes revealed a hexagonal lattice of the same lattice constant, formed by surface-exposed parts of the LHC molecules which are probably responsible for the ordered stacking of lattices. In both the LHC aggregates and in the reconstituted membrane lattices the diffracted x-ray intensities at 10-A spacing on the equator indicate that the LHC molecule contains paralled alpha-helices or beta-sheets that are oriented perpendicular to the planar arrays.