Many soil properties, including microbial activity, depend on the types and amounts of clay minerals present. Consequently, the persistence of the toxin from B.thuringiensis subsp. kurstaki in soils with different clay mineralogies and other physicochemical properties and in these soils amended with various amounts of montmorillonite or kaolinite or with CaCO3 was studied by insect bioassays, in an attempt to determine the properties that affect the persistence of these insecticidal toxins in soil.