The nucleic acid of infectious bronchitis virus (IBV), like that of other enveloped viruses, consists of discontinuous single stranded RNA. However, unlike many other viruses, there is extreme heterogeneity in the sizes of the RNA fragments, as revealed by centrifugation in sucrose gradients or electrophoresis in polyacrylamide gels. Two principal classes of RNA fragments are present: a. A larger class comprising 74.9–85.4 per cent of total RNA and consisting of fragments having molecular weights ranging from 0.5×10(6) to considerably greater than 3.0×10(6) daltons and, b. A smaller class comprising 9.1–19.7 per cent of total RNA with the size approximately that of ribosomal 4S RNA. All IBV RNA's were fully susceptible to ribonuclease and had a buoyant density in caesium sulphate identical to that of tobacco mosaic virus RNA. No difference in the RNA profile for IBV was observed from the use of different methods of virus purification. The single-stranded RNA's of poliovirus and tobacco mosaic virus remained undegraded after preparation in the presence of IBV.