Bluetongue virus (BTV), a representative of the orbivirus genus of the Reoviridae, is considerably larger (at 80 nm across), and structurally more complex, than any virus for which we have comprehensive structural information. Orbiviruses infect mammalian hosts through insect vectors and cause economically important diseases of domesticated animals. They possess a segmented double-stranded RNA genome within a capsid composed of four major types of polypeptide chains. An outer layer of VP2 and VP5 is removed as the virus enters the target cell, to leave an intact core within the cell. This core is 70 nm across and composed of 780 copies of VP7 (M(r) 38K) that, as trimers, form 260 'bristly' capsomeres clothing an inner scaffold constructed from VP3 (M(r) 103K). We report here the crystal structure of VP7 from BTV serotype 10, which reveals a molecular architecture not seen previously in viral structural proteins. Each subunit consists of two domains, one a beta-sandwich, the other a bundle of alpha-helices, and a short carboxy-terminal arm which might tie trimers together during capsid formation. A concentration of methionine residues at the core of the molecule could provide plasticity, relieving structural mismatches during assembly.