Membranous cytochrome oxidase forms a regular two-dimensional array, as visualized by negative staining and electron microscopy. The lattice formed by the spots in the micrographs, which are assumed to be the cytochrome oxidase protein complexes, belongs to the two-dimensional rectangular space group pg; the unit cell dimensions are 88 ± 4 × 127 ± 7 A ̊ , with two protein complexes per unit cell. By using the unit cell area in conjunction with the chemical composition of the membrane and the molecular weights and volumes of the constituents, it was possible to construct a geometrical model for the membrane which quantitatively accounts for the measurements in hand. The lipids are present as lamellar bilayer, and the protein complexes pass all the way through the bilayer.