1. The effect of ubiquinone (CoQ(9)) on ascites in broiler chickens was investigated. 2. The commercial broilers were divided into 2 groups of 100 birds each; CoQ(9) treated group and non-treated group. 3. The chickens were grown in a positive-pressured house with double high efficiency particulate air filtered intakes and exhaust, and thus were strictly isolated from infectious agents. 4. The chickens (15 to 21 d old) were exposed to cold stress in order to induce ascites. 5. The number of birds with ascites in the CoQ(9)-treated group was significantly lower than in the non-treated group. 6. Survival and production rates were better in the CoQ(9)-treated group than in the non-treated group.