The ultrastructure of distemper virus was studied in the electron microscope by use of the negative staining technique. The structure was shown to be generally similar to that of measles, rinderpest and large myxoviruses. The intact virus particles measured 1150 to 2300 a in diameter and a marked pleomorphism of particles was observed. The marginal structure was constituted by a double-contoured membrane with a width of 75–85 a from which extended rather symmetrically arranged club-shaped surface projections with a length of 130 a. An inner component with a probably helical symmetry and a cross-section diameter of 180 a was demonstrable.