Adsorption, desorption and insecticidal activity of 65 kDa and 130 kDa proteins from Bacillus thuringiensis subsp. Kurstaki (Btk) on red soil colloids were studied. The proteins from Btk were easily adsorbed on red soil colloids and the adsorption reached equilibrium within 0.5 h The adsorption rate of 65 kDa protein was higher than that of 130 kDa protein The adsorption isotherms of the two proteins in Tris buffer (pH 8) followed the Langmuir equation (R² > 0.992) and more 65 kDa proteins adsorbed than 130 kDa proteins. The adsorption rates of the two proteins decreased slightly with the rising temperature from 10�C to 50�C, and declined, too, with the rising pH value from 6 to 8 (Tris buffer). Both free and adsorbed proteins were toxic to the larvae of Heliothis armigera, and the insecticidal activity of adsorbed proteins was 2~5 times higher than that of free proteins. Desorption experiments revealed that the proteins were tightly bound onto the clays, and only 37.2% and 22.9% of the proteins, 65 kDa and 130 kDa, respectively, adsorbed at equilibrium was desorbed after three washes with deionised water, 20.7% and 14.8% was desorbed after three washes with Tris buffer (pH 8). Infrared absorption spectra showed that the proteins did not alter much in structure, either adsorbed or desorbed. The analysis using transmission electron microscopy (TEM) showed that no obvious changes was observed in particle size of soil colloids In red soil either before or after adsorption. Adsorbed Bt proteins were higher in insecticidal activity, but low in desorption rate, which means that release of proteins from transgenic Bt plants may pose a potential environmental risk.