Eight strains of Newcastle disease virus (NDV) have been examined using the technique of negative contrast electron microscopy, before and after treatment with ether. There is considerable variation in the shape and size of particles in any one strain, and the degree of this pleomorphism varies from strain to strain. The two common features of all the strains are the internal ribonucleoprotein and the rosette-like haemagglutinin formed by disruption of the coat with ether. The effect of ether is to disrupt the particles, but the ease of disruption varies from strain to strain. The products of ether treatment are the internal ribonucleoprotein and the haemagglutinin, which consists of portions of the outer coat. Besides these, the coat itself may in some strains appear almost intact, but stripped of its projections, after ether treatment. With the strains whose pathogenicity was known it has not proved possible to relate structure to degree of pathogenicity.