A virus was isolated from a natural population of larvae of Culicoides sp. The viral particles were naked, icosahedral, with an electron microscopic diameter of 54 nm. They banded at a density of 1.32 in CsCl. The genome consisted of two pieces of dsRNA with molecular weights of 2.5 and 2.6×106 daltons. The proteins were resolved into six polypeptides by polyacrylamide gel electrophoresis. According to these characteristics, this Culicoides virus is akin to infectious pancreatic necrosis virus, infectious bursal disease virus, Drosophila X virus, tellina virus and ostrea virus. In this group, the Culicoides virus and the Drosophila X virus are closely related. © 1983, All rights reserved.