The adsorption of the insecticidal Cry1Aa protein from Bacillus thuringiensis (Br-toxin) on a model clay surface was studied to understand the structural changes of the protein induced by the clay surface. We studied the adsorption of the monomeric and soluble oligomeric forms of the Cry1Aa toxin as a function of pH and ionic strength conditions on montmorillonite, which is an electronegative phyllosilicate. Cry1Aa secondary structure was determined from the amide I' FTIR absorption profiles. Accessibility to the solvent was determined by NH/ND exchange to characterize conformational flexibility of the different states of the Cry1Aa protein. The size distribution of Cry1Aa solutions was obtained by dynamic light scattering (DLS). From combined DLS and FTIR measurements, we conclude that montmorillonite traps the Cry1Aa toxin in its monomeric state, preventing the oligomerization of the protein. The oligomeric forms were adsorbed onto the clay without significant structural changes.