The structure of the core of bluetongue virus has been determined to 3,5 Å resolution by x-ray crystallography. Solving such a structure composed of 1 000 proteins is a challenge. It necessitates close collaborations between crystallographers, virologists and electron microscopists. The core is made of a proteinic capsid, 700 Å in diameter, enclosing a double-stranded RNA genome and a set of proteins required for transcription. The crystallographic structure reveals how the proteins of the capsid self-assemble, to form 2 layers whose subunits are arranged on different quasi-equivalent lattices. Tubes of electron density are observed below this envelope, which can be assigned to the RNA genome.