The intact particles and nucleocapsids of mumps, Sendai and measles viruses are of closely similar appearance, size and structure. The intact particles are about 150 nm. in diameter. The filamentous nucleocapsids have a modal length of about 1.1 µm., and are constructed of subunits arranged as a single start helix of pitch 5.0 nm. for Sendai virus, and about 6.0 nm. for mumps and measles viruses. A (helical) projection of the structure of the Sendai nucleocapsid calculated from an electron micrograph showed that the structural subunits are hour-glass-shaped and are arranged in the helix with their long axes inclined at an angle of about 60° to the long axis of the particle. There are probably 11 or 13 subunits in each turn of the basic helix. Optical diffraction patterns of electron micrographs of mumps and measles nucleocapsids show that they have closely similar structures.