The ability of lentogenic strains of Newcastle disease virus (NDV) to inhibit the development of embryonic chicken muscle cells in vitro is described. In contrast to the uninfected control, which displayed a thick network of branched multinucleated muscle fibers, cultures infected with NDV exhibited a severely reduced potential to differentiate. The few differentiated muscle fibers that formed in the presence of NDV were poorly developed, displayed an attenuated morphology, and detached from the substrate 2 to 3 days before uninfected cultures showed signs of cell aging. When NDV-infected cultures were stained before muscle fiber detachment with a fluorescent-labeled NDV-specific antibody, the infected muscle fibers reacted strongly relative to neighboring myoblasts and fibroblasts. All six strains of NDV induced a similar inhibitory effect, and no strain-specific differences were detectable. These results indicate that differentiating muscle fibers are highly susceptible to infection by NDV in vitro and that this tissue exhibits an early NDV-induced cytopathic effect.