Larvae of the pine emperor moth, Nudaurelia cytherea capensis, infected with Nudaurelia beta virus (N beta V) consistently contained one species of dsRNA. This dsRNA was the correct size to be the replicative form of the N beta V genome and, in Northern blots, it hybridized with 32P-end-labelled virion RNA. Other smaller dsRNAs were obtained in a non-reproducible manner but these had no sequences in common with the genomic probe; no dsRNAs were observed in extracts from virus-free larvae. Cell-free translation of N beta V RNA resulted in the synthesis of only one major polypeptide, of Mr about 71,000, which could not be precipitated by an anti-N beta V serum.