Bluetongue virus particles, converted to a high density form by the selective removal of two polypeptides from their protein capsids, possess RNA dependent RNA polymerase activity. The enzyme, which can be assayed by its ability to incorporate nucleoside triphosphates into RNA in an in vitro system, is dependent on magnesium ions, is stimulated by the presence of manganese ions and shows maximal activity at 28°C. The product of the in vitro reaction was isolated and shown to consist of ten single-stranded RNA segments which can be hybridized with double-stranded RNA isolated from purified bluetongue virus (BTV). The hybridization product, when analyzed by means of polyacrylamide gel electrophoresis, is indistinguishable from a hybrid obtained using BTV messenger RNA isolated from infected cells. It is therefore deduced that the BTV genome is fully transcribed both in vitro and in vivo by an enzyme present in the viral capsid.