Understanding the mechanisms of action of membrane proteins requires the elucidation of their structures to high resolution. The critical step in accomplishing this by x-ray crystallography is the routine availability of well-ordered three-dimensional crystals. We have devised a novel, rational approach to meet this goal using quasisolid lipidic cubic phases. This membrane system, consisting of lipid, water, and protein in appropriate proportions, forms a structured, transparent, and complex three-dimensional lipidic array, which is pervaded by an intercommunicating aqueous channel system. Such matrices provide nucleation sites (''seeding'') and support growth by lateral diffusion of protein molecules in the membrane (''feeding''). Bacteriorhodopsin crystals were obtained from bicontinuous cubic phases, but not from micellar systems, implying a critical role of the continuity of the diffusion space (the bilayer) on crystal growth. Hexagonal bacteriorhodopsin crystals diffracted to 3.7 Å resolution, with a space group P6 3 , and unit cell dimensions of a ‫؍‬ b ‫؍‬ 62 Å, c ‫؍‬ 108 Å; ␣ ‫؍‬ ␤ ‫؍‬ 90؇ and ␥ ‫؍‬ 120؇.