Abstract: Acute toxicity to silkworm (Bombyx mori L.) caused by chlorpyrifos was tested.Residue dynamic of chlorpyrifos was determined on mulberry leaf sprayed with 40% chlorpyrifos EC diluted by 1 000 or 1 500 times with water.The acute toxicity and sub-lethal effect caused by the chlorpyrifos residues to silkworms was also investigated.As a result,the 96 h LC50 of chlorpyrifos to silkworms was 0.82 mg／kg of mulberry leaf with feeding method.According to the toxicity category,the acute toxicity of chlorpyrifos to silkworm belongs to the high class.The results also showed that when applied in a rate of high dose,the short half-life of the dissipation rate of chlorpyrifos in mulberry leaves was 1.82 ~3.88 d.At the 22nd day after the application,the residue of chlorpyrifos was 0.01 ~0.28 mg／kg in mulberry leaf.When applied with chlorpyrifos diluted by 1 500 times,the residue was found to be at the level of LOD ~0.20 mg／kg of mulberry leaf at the 18th day.When feeding with the dose of 0.56 mg／kg of mulberry leaf,the silkworms showed no observed effect.When feeding with a dose lower than 0.20 mg／kg of mulberry leaf,the mortality,larval weight,cocoon weight and cocoon shell ratio did not decrease significantly as compared with that of the control treatment.In conclusion,in order to prevent the harmful effects to silkworms,the application of 40% chlorpyrifos EC diluted by 1 500 times within one application in the whole season was recommended with pre-harvest interval of 18 days as the GAP.