The nucleic acids of influenza A, B, and C viruses were compared. Susceptibility to nucleases demonstrates that influenza C virus, just as influenza A and B viruses, possesses single stranded RNA as its genome. The base compositions of the RNAs of influenza A, B, and influenza C virus are almost identical and comparative analysis on polyacrylamide gels shows that the genome of influenza C/GL/1167/54 virus, like that of the RNAs of influenza A and B viruses, is segmented. Eight distinct RNA bands were found for influenza A/PR/8/34 virus and for influenza B/Lee/40 virus. The RNA of influenza C/GL/1167/54 virus separated into at least four segments. The total molecular weights of the RNA of influenza A/PR/8/34 and B/Lee/40 virus were calculated to be 5.29 x 106 and 6.43 x 106, respectively. A minimum value of 4.67 x 106 daltons was obtained for influenza C/GL/1167/54 virus RNA. The data suggest that influenza C viruses are true members of the influenza virus group.