Molecular dynamics simulations reveal substructures within the liquid-ordered phase Of lipid bilayers. These substructures, identified in a 10 mu s all-atom trajectory of liquid-ordered/liquid-disordered coexistence (L-o/L-d) are composed of saturated hydrocarbon chains packed with local hexagonal order and separated by interstitial regions enriched in cholesterol and unsaturated chains. Lipid hydrocarbon chain order parameters calculated from the L-o phase are in excellent agreement with H-2 NMR measurements; the local hexagonal packing is also consistent with H-1-MAS NMR spectra of the L-o phase, NMR diffusion experiments, and,small-angle X-ray and neutron scattering. The balance of cholesterol-rich to local hexagonal order is proposed to control the partitioning of membrane components L-o regions. The latter have frequently associated with formation of so-called rafts, platforms in the plasma membranes of cells that facilitate interaction between components of signaling pathways.