Ruska and Kausche1 and Farrant and Fenner2 demonstrated a close similarity, both in size and superficial morphology, between myxoma and vaccinia viruses. This, together with the demonstration of similarity in the pathogenesis of myxomatosis and other pox virus diseases, was considered by the latter authors to justify the inclusion of myxoma virus within the pox group. Further justification for this view has now been obtained by a comparison of the morphology and fine structure of myxoma and vaccinia viruses using the negative contrast technique of Brenner and Horne3.