Chemical reactions and fate of the toxins of Bacillus thuringiensis(Bt) in the soil environment are causing increasing concerns due to the large-scale cultivation of transgenic Bt plants.In this study,the effect of ionic strength(0-1 000 mmol kg 1) adjusted by NaCl or CaCl 2 on adsorption of Bt toxin by a lateritic red soil,a paddy soil and these soils after chemical removal of organic-bound or free Fe and Al oxides,as well as by pure minerals(goethite,hematite and gibbsite) which are widespread in these soils,were studied.The results indicated that when the supporting electrolyte was NaCl,the adsorption of Bt toxin by the lateritic red soil and paddy soil increased rapidly until the ionic strength reached 250 mmol kg 1 and then gradually slowed down with the increase of ionic strength;while in case the supporting electrolyte was CaCl 2,the adsorption of Bt toxin enhanced significantly at low ionic strength(< 10 mmol kg 1) and then decreased as the ionic strength increased.The adsorption of Bt toxin by the tested minerals and soils after the removal of organic-bound or free Fe and Al oxides also increased with increasing ionic strength controlled by NaCl.Removing organic-bound Fe and Al oxides obviously increased the adsorption of Bt toxin in the tested soils.Differently,removing free Fe and Al oxides increased the Bt adsorption by the paddy soil,but decreased the adsorption by the lateritic red soil.The study indicated that the varieties of ionic strength and the presence of Fe and Al oxides affected the adsorption of Bt toxin by the soils,which would contribute to the further understanding of the fate of Bt toxin in the soil environment and provide references for the ecological risk assessment of transgenic Bt plants. Chemical reactions and fate of the toxins of Bacillus thuringiensis (Bt) in the soil environment are causing greater concern due due to the large-scale cultivation of transgenic Bt plants. In this study, the effect of ionic strength (0-1 000 mmol kg 1 ) adjusted by NaCl or CaCl 2 on adsorption of Bt toxin by a lateritic red soil, a paddy soil and these soils after chemical removal of organic-bound or free Fe and Al oxides, as well as by pure minerals (goethite, hematite and gibbsite ) which are widespread in these soils, were studied. The results indicated that when the supporting electrolyte was NaCl, the adsorption of Bt toxin by the lateritic red soil and paddy soil increased sharp until the ionic strength reached 250 mmol kg 1 and then gradually slowed down with the increase of ionic strength; while in the case of supporting electrolyte was CaCl 2, the adsorption of Bt toxin enhanced significantly at low ionic strength (<10 mmol kg 1) and then decreased as the ionic strength increased. on of Bt toxin by the tested minerals and soils after the removal of organic-bound or free Fe and Al oxides also increased with increasing ionic strength controlled by NaCl.Removing organic-bound Fe and Al-oxides obviously increased the adsorption of Bt toxin in the tested soils. Differently, removing free Fe and Al oxides increased the Bt adsorption by the paddy soil, but decreased the adsorption by the lateritic red soil. The study indicated that the varieties of ionic strength and the presence of Fe and Al oxides affected the adsorption of Bt toxin by the soils, which would contribute to the further understanding of the fate of Bt toxin in the soil environment and provide references for the ecological risk assessment of transgenic Bt plants.