The structure of the bluetongue virus (BTV) particle, determined by cryoelectron microscopy and image analysis, reveals a well-ordered outer shell which differs markedly from other known Reoviridae. The inner shell is known to have an icosahedral structure with 260 triangular spikes of VP7 trimers arranged on a T = 13,l lattice. The outer shell is seen to consist of 120 globular regions (possibly VP5), which sit neatly on each of the six-membered rings of VP7 trimers. "Sail"-shaped spikes located above 180 of the VP7 trimers form 60 triskelion-type motifs which cover all but 20 of the VP7 trimers. These spikes are possibly the hemagglutinating protein VP2 which contains a virus neutralization epitope. Thus, VP2 and VP5 together form a continuous layer around the inner shell except for holes on the 5-fold axis.