Acute inhalation intoxication of CS (O-chlorobenzylidene malononitrile) occurred among the 192 animals in confined animal cages of farm as the result of prolonged exposure. A total of 8 animals (3 silver foxes, 3 fitches and 2 minks) died in 15 hours after the exposure. Distinct evidences of pulmonary atelectasis were observed as with hepatorenal damages. The lethal toxicity of CS was considered to be due to early severelung damages leading to asphyxia, accompanying acute toxic hepatitis and nephritis.