The adsorption of the toxin from Bacillus thuringiensis (Bt-toxin), which is synthesized in genetically modified maize, on sterilized Na-montmorillonite and on H2O2-treated and untreated clay fractions of three soils from different sites were studied. All adsorption isotherms can be described by a linear isotherm. Although all clay fractions from the different soils show nearly the same mineralogical composition, we found different affinities ranging from k = 47.7 to k = 366.7 of the adsorbates for the Bt-toxin. The H2O2-treated clay fractions show no correlation between the adsorption affinity and the amount of soil organic matter. On the other hand, there is a correlation between the content of organic carbon and the adsorption affinity of the untreated clay fractions. This can be explained by the fact that due to the coatings of soil organic matter on aggregates, the Bt-toxin polymers are not able to adsorb within the clay aggregates.