Crystals of a chlorophyll-protein from Chloropseudomonas ethylicum were studied by X-ray diffraction and electron microscopy. Dimensions of the unit cell (hexagonal space group P6 3) are a = b = 195 ± 1 A ̊ and c = 98.4 ± 0.5 A ̊ . The unit cell contains six macromolecules (120 chlorophylls). Crystals fixed with glutaraldehyde were sectioned either perpendicular or parallel to the crystal ( c) axis and examined in an electron microscope. Sections cut parallel to the crystal axis showed alternate lightdark striations parallel to the axis with a repeat distance of 106 ± 16 Å. Some sections cut perpendicular to the axis showed an hexagonal array of electron-transparent “holes” 120 ± 20 Å apart. The holes and light striations are thought to indicate channels parallel to the c axis at each corner and through the center of each hexagonal unit cell A macromolecular shape approximating a prolate ellipsoid, 88 Å long and 65 Å in diameter, is consistent with the volume of the macromolecule and crystalline packing considerations. Electron micrographs of negatively stained individual macromolecules indicate an average diameter of 67 ± 13 Å.