RNA polymerase associated with cytoplasmic polyhedrosis virus, which contains double-stranded RNA as a genome, synthesizes single-stranded RNA in vitro. The RNA product has the same size distribution to the genome segments, showing two fractions on a glycerol concentration gradient centrifugation. Each fraction of the synthesized RNA hybridized specifically with the corresponding size fraction of the denatured genome RNA segments. Only after the transcription has been carried out over the whole length of each genome segment, the single-stranded RNA product leaves the virus particle. It seems that the transcription of every segment starts at the same time, although the transcribed RNA for the shorter segment group is released from a virion faster than the larger group.