The structure of the core particle of bluetongue virus has been determined by X-ray crystallography at a resolution approaching 3.5 Angstrom. This transcriptionally active compartment, 700 Angstrom in diameter, represents the largest molecular structure determined in such detail. The atomic structure Indicates how approximately 1,000 protein components self-assemble, using both the classical mechanism of quasi-equivalent contacts, which are achieved through triangulation, and a different method, which we term geometrical quasi-equivalence.