In this study, the 22 different commercial silkworm races in Hunan province during 4th instar larvae were fed continuously on mulberry leaves immersed in serial two-fold dilutions of fluoride solutions with original concentration of 150, 300, 600, 1 200, 2 400 mg/kg respectively. The toxicological regression line(LDP),median lethal concentration (LC50 ), 95％ fiducial limit of LC50, relation coefficient(r) and chi square test( χ2) were performed by a method according to the probit analysis and the least square．The bioassay result showed that there was distinct fluoride resistibility among these different commercial silkworm races in Hunan province. The commercial silkworm races “1514”, “Qiubai B”, “8535”, “Xiafang B” and “Qiufeng B” were highly tolerant to fluoride, with LC50′s greater than that of silkworm strain “T6”, which harbor the fluoride tolerance major gene, and the commercial silkworm races “Yun 1”, “Yun 2”, “Haoyue A” and “Ri 3” were highly sensitive to fluoride with LC50′s less than 1 000 mg/kg.