Abstract: Fipronil is one of the insecticides popularly used in paddy rice and mulberry mix-cultivation area in China. Toxicity of fipronil to silkworm(Bombyx mori) in different situations were conducted by methods of feeding toxic mulberry leaves and injecting into the mouthparts, and methods of contacting with toxic membrane, respectively. For the second,third,fourth and fifth-instar larvae, the LC50 value(48 h) of fipronil to silkworm was 4.89,6.46,9.47 and 11.59 mg/L,respectively. The LD50 value for fifth instar was 1.38 μg/silkworm. When environmental temperature was 25℃,30℃ and 35℃, the LC50 value (48 h) to the second-instar larvae was 4.81,4.59 and 2.05 mg/L, respectively. When immersion time of mulberry leaves in fipronil solution was 1 s,10 s,1 min,10 min and 1 h, the LC50 value (48 h) to the third-instar larvae was 9.12,6.46,4.39,3.68 and 2.80 mg/L, respectively.After contact of silkworm with filter paper treated by fipronil solution in time of 1,10,30 and 60 min, the LC50 value (48 h) to the fourth-instar larvae was 2.82,2.35,2.05 and 1.57 μg/cm2 ,respectively.The results indicated that the toxicity of fipronil to silkworm is caused mainly by mode of feeding and the risk of fipronil to silkworm is relatively high when fipronil is used near mulberry cultivation area.