The plaque technique is applied to the type differentiation of the Newcastle disease viruses, 0.5 per cent agarose being used as a nutritive overlayer. The epizootic Newcastle disease strains are characterized by a heterogenic plaque population in which the large plaques are prevalent, averaging 3--4 mm, with an irregular round form, an unclearly delineated diffuse edge, and a red center. The vaccinal strains (B1 and K) yield a homogeneous small-plaque population, the plaques themselves being 1--2 mm large, with a regular round shape, a well delineated edge, and a bright center.