The early stages of adaptation in chick embryos of the BON strain of influenza B virus were characterized by Burnet, Beveridge, and Bull.1 No fluids giving fowl/guinea pig (F/GP) hemagglutinating ratios below unity were observed, but with allantoic passage high F/GP ratios were obtained.1 Burnet and his colleagues concluded that the changes in the F/GP ratio were not sufficiently marked to allow any definition of O and D forms analogous to those obtained with influenza A.2'3 Francis, Salk, and Brace4 working with recently isolated virus have shown that strains of influenza B virus agglutinate fowl cells to high titer. In view of the fact that several strains of adapted influenza B virus showed a F/GP ratio of 10 or 20 to 1, it was decided to reinvestigate the possibility that there were distinct phases. This report presents evidence which indicates that there are two