The release of organisms that have been genetically modified to contain the genes that code for the insecticidal toxins of subspecies of B. thuringiensis may have adverse effects on the environment.  To evaluate such potential effect, the adsorption and binding of these toxins on clay minerals and other soil particulates and the effects of such surface interactions on the persistence and activity of the toxins must be established.  The equilibrium adsorption and binding of the toxins from B. thuringiensis ssp kurstaki,  toxic to lepidopteran larvae, and from ssp tenebrionis, toxic to coleopteran larvae, on the clay minerals, montmorillonite and kaolinite, homoionic to various cations ('clean' clays) or coated with two types of polymeric oxyhydroxides of Fe(III) ('dirty' clays) were studied.  Adsorption of the toxins on a constant amount of the clays increased with toxin concentration and then reached a plateau.  Larger amounts of the toxins from Btk  than from Btt were adsorbed.  Adsorption of the toxins was rapid and maximal between pH 6 and 8 onto clean clays and between pH 5 and 9 onto dirty clays.  Adsorption of the toxins from Btk or Btt on clean clays was affected by the type of cation to which the clays were homoionic.