In order to evaluate the hazard of chlorpyrifos to the cultured shrimp Litopenaeus vannamei,the lethal effect of the shrimp infected with white spot syndrome virus(WSSV)under the chlorpyrifos stress was tested,while the quantity of WSSV in the gill tissue and acetylcholinesterase acticity in the muscle tissue were analyzed.The 96 h median lethal concentration(LC₅₀)of chlorpyrifos to the shrimp was determined.The results showed that the 96 h LC₅₀ is 0.758 μg/L(0.521-0.987 μg/L),and as time prolonged,the values of LC₅₀ significantly declined,which showed a positive concentration-response relationship.On this basis,the stress concentration of chlorpyrifos of 0.2 μg/L was determined.After the shrimp were exposed to 0.2 μg/L chlorpyrifos for 4 days,they were infected with WSSV by injection.It showed that the cumulate mortality of chlorpyrifos-WSSV treatment(83.33?Ee4.7%)was significantly higher than that of ethanol-WSSV group(40.00?Ee0.9%).Meanwhile,the amount of WSSV was also determined by Real time Relative PCR at 48,72 and 96 h post injection.WSSV quantity of chlorpyrifos-WSSV treatment was about 4 times that of ethanol-WSSV control at 72 h.WSSV quantity of chlorpyrifos-WSSV treatment was significantly increased at 96 h,which was 4.9 times that of chlorpyrifos-WSSV treatment at 72 h,and 5.9 times that of ethanol-WSSV treatment at 96 h.The acetylcholinesterase(AchE)activity in shrimp muscle tissue under chlorpyrifos stress was 20% lower than that in control group.This showed that the proliferation rate of WSSV in shrimp increased rapidly under chlorpyrifos stress,which raised the mortality rate of the shrimp.