The freeze-etching technique when applied to FV3 suspensions revealed the perfectly icosahedral structure of the viral nucleocapsids. This allowed a fine substructure suggesting the existence of numerous subunits of small dimensions to be detected at their surface. Observations of the replicas of infected cells obtained by freeze-etching revealed modifications of the external face of the cytoplasmic membrane occurring at the level of the budding viral particles; in these zones parallel stripes appeared, whose elementary structure is constant (13,5 nm). Observations of the infected cells with the scanning microscope enabled the spreading of the budding process on all the cells of a same culture to be studied. By counting the number of budding particles per mum2, it was possible to demonstrate the heterogeneity of the budding process at the surface of the cells.