Abstract The radial electron density distribution in the Sindbis virus particle has been determined to a resolution of 28 A from measurements of spherically averaged X-ray diffraction. The most striking feature of the density profile is a deep minimum at r = 232A, from which we infer that the lipids of Sindbis virus are organized in a bilayer at about that radius. Comparison of these results with electron micrographs of intact virus and of isolated cores shows that the inner polar groups of the bilayer interact directly with the core protein, and the outer polar groups with the external glycoprotein. Protein probably does not form a bridge across the bilayer.