Growth inhibitory activity has proven important in Qiufeng × Baiyu and Qingsong × Haoyue silkworm larvae. The consumption of mulberry leaves was reduced in both silkworm races while Qiufeng × Baiyu larvae showed the higher reduction in leaf consumption. From the results obtained, it was revealed that even at low concentrations of 400 mg/L of either Cr( Ⅲ ) or Cr(Ⅵ)ions, growth of Qiufeng × Baiyu and Qingsong × Haoyue was significantly depressed.Depression in relative growth index(RGI) and high death rate in both silkworm races indicated that the different concentrations of the two ions used caused their growth inhibitions. Through linear regression analysis, the EC5o (concentration of the compound that caused 50% reduction) was interpolated for both tested compounds. EC50(mg/L) of Cr ( Ⅲ ) ions in Qiufeng × Baiyu and Qingrs ong × Haoyue were 800 and 600 respectively. EC50(mg/L) of Cr( Ⅵ ) ions in Qiufeng × Baiyu and Qingsong × Haoyue was 600 and 316 respectively.According to the analysis of relative growth index, and an analysis of linear regression technique for measuring the growth of the silkworm races, it was indicated that the form of Cr affected growth rates, growth inhibition responses of the larvae, and toxicological effects. Thus, form and quantity of Cr accumulating in silkworms reared with contaminated leaves are likely to influence their population dynamics.