Purified epizootic haemorrhagic disease virus (EHDV) was shown to contain 10 double-stranded RNA segments and a double-layered protein capsid with 4 major and 4 minor polypeptides. The virus differed from bluetongue virus (BTV), the orbivirus prototype, in that EHDV had an additional minor polypeptide component. This component, together with the major polypeptides P2 and P5, formed the outer capsid layer of the virus. The extra polypeptide apparently stabilizes this layer since, unlike BTV, EHDV was quite stable on CsC1 gradients at both pH 7,0 and 8,0. EHD virions were found to have a density of 1,36 g/mℓ, while particles without the outer capsid layer were isolated and had a density of 1,40 g/mℓ. Two non-capsid polypeptides, P5A and P6A, were identified in addition to the 8 capsid polypeptides. Polypeptide P5A was synthesized in excess of all the others. There was little homology between the nucleic acids of EHDV and BTV with only 5-10% cross-hybridization. No hybrid double-stranded RNA segments were identified. We found by cross-immune precipitation that the major core polypeptides of the 2 viruses (P7 and P3) have common antigenic determinants.