Seven strains of influenza A2 virus were serially passed in the chick embryo, and morphological characteristics of the passages were examined in the electron microscope. With serial passage there was a change from a prominently filamentous appearance in early passages to an almost entirely spherical appearance in later passages. The number of passages required for the conversion to spherical morphology varied with different strains. The filament-sphere variation was found to be independent of the properties which differentiate "+" and "-" influenza A2 virus particles, and both highly filamentous and spherical populations of "+" and of "-" virus particles were obtained. The usefulness of these pairs of independent markers for genetic studies is discussed.

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