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A sudden increase in mortality, preceded by a short history of respiratory signs and diarrhea, occurred in a backyard flock of 48 game chickens in the Central Valley of California. Necropsy findings included severe generalized linear hemorrhages and/or ulcers in the digestive tract, larynx, and trachea. Histology revealed severe multifocal hemorrhages and necrosis in the mucosa of the respiratory and digestive tracts, vasculitis, and necrosis of lymphoid tissue. The birds were serologically negative to Newcastle disease virus; this was consistent with an acute infection. The avian paramyxovirus type 1 isolated was characterized as velogenic viscerotropic Newcastle disease virus. A thorough epidemiologic investigation was carried out, and no other premises were found to have birds with clinical signs or evidence of exposure. The entire outbreak was limited to the original backyard flock and resolved within 14 days of the onset of clinical signs.