Summary Poly (A) segments form a part of mammalian mRNA (Darnell, et al. 1971; Edmonds, Vaughan & Nakazato, 1971; Mendecki, Lee & Brawerman, 1972), vaccinia virus mRNA (Kates, 1970) and the RNA of a number of RNA viruses (Armstrong et al. 1972; Gillespie, Marshall & Gallo, 1972; Johnston & Bose, 1972; Lai & Duesberg, 1972). The potential significance of the poly (A) segment in mRNA function is unknown, but it is of interest that the genomes of RNA tumour viruses contain a much greater proportion of poly (A) than the RNAs of other viruses and that poly (A) segments have not been detected in virus RNAs that do not function directly as mRNA, such as the RNAs of influenza virus, Newcastle disease virus and vesicular stomatitis virus (Gillespie et al. 1972; Lai & Duesberg, 1972). It has been reported that the poly (A) segment of poliovirus RNA represents 0.75% (Armstrong et al. 1972) or 0.25% (Gillespie et al. 1972) of the total nucleotide content of the virus genome.