The ultrastructure of CV-1 cells infected with subacute sclerosing panencephalitis (SSPE) viruses was compared with that of CV-1 cells infected with the wild or Edmonston strain of measles virus. Both SSPE viruses and the measles viruses produced two types of nucleocapsid structures: smooth filaments, 15 to 17 nm in diameter, and granular filaments, 22 to 25 nm. The smooth and granular filaments produced by SSPE and measles virus did not differ in appearance. In CV-1 cells infected with SSPE viruses, smooth filaments formed large intranuclear inclusions and granular filaments occupied a large area of the cytoplasm, but always spared the area under the cell membrane. Particles budding from the surface of these cells contained no nucleocapsids. In CV-1 cells infected with measles virus, only small aggregates of smooth filaments were seen in the nuclei. Granular filaments in the cytoplasm predominantly occupied the area under the cell membrane, and were aligned beneath the cell membrane in a parallel fashion and assembled into budding particles. These differences between SSPE and measles virus may be regarded as quantitative, but they do distinguish SSPE viruses from measles virus. Moreover, the formation of large nuclear inclusions filled with smooth filaments appears to be a characteristic process of SSPE, but not of measles, since this type of inclusion is invariably seen in SSPE brain tissues, brain cultures derived from them, and CV-1 cells infected with SSPE viruses.