In FHM and RTG-2 cells infected with IPN virus two types of viral directed RNAs where isolated in the presence of 0.5 mug/ml actinomycin D. One type, sedimenting at 14-15 S in sucrose gradients, was shown to be resistant to digestion by pancreatic RNase and did not precipitate in presence of lithium chloride 2 M. Furthermore the molecules sedimented in the same way as the double stranded RNA extracted from purified virions. The second type of RNA, sedimenting at 23-24 S, was sensitive to RNase and was precipitable by LiCl, indicating that it was single-stranded. Additionally it was shown by polyacrylamide gel electrophoresis that this type of RNA resolved into four peaks. The single stranded 23-24 S RNA was detectable in cells from the third hour following infection; it represents the major portion of total viral directed RNA that was synthesized during a cycle of replication. The synthesis of both single stranded and double stranded RNA increased exponentionally until the 7th hour post-infection and began to decrease at about 10 hours.