The base sequence homology between all 8 32P-labeled RNA segments of fowl plague virus (FPV) and the complementary (c) RNA of an influenza B virus (B-Mass), and between segment 8 of virus N and the cRNA of the same influenza B strain was determined. All segments of FPV and segment 8 of virus N show a significant base sequence homology, ranging from 18-50% suggesting that influenza A and B viruses have a common ancestor. The conserved regions in segments 4,6 and 8 of the influenza A strains are identical to corresponding regions of the influenza B virus tested.