Insecticidal proteins produced by various subspecies (kurstaki, tenebrionis, and israelensis) of Bacillus thuringiensis (Bt) bound rapidly and tightly on clays, both pure mined clay minerals and soil clays, on humic acids extracted from soil, and on complexes of clay and humic acids. Binding reduced susceptibility of the proteins to microbial degradation. However, bound proteins retained biological activity. Purified Cry1Ab protein and protein released from biomass of transgenic Bt corn and in root exudates of growing Bt corn (13 hybrids representing three transformation events) exhibited binding and persistence in soil. Insecticidal protein was also released in root exudates of Bt potato (Cry3A protein) and rice (Cry1Ab protein) but not in root exudates of Bt canola, cotton, and tobacco (Cry1Ac protein). Vertical movement of Cry1Ab protein, either purified or in root exudates or biomass of Bt corn, decreased as the concentration of the clay minerals, kaolinite or montmorillonite, in soil increased. Biomass of transgenic Bt corn decomposed less in soil than biomass of near-isogenic non-Bt corn, possibly because biomass of Bt corn had a significantly higher content of lignin than biomass of non-Bt corn. Biomass of Bt canola, cotton, potato, rice, and tobacco also decomposed less than biomass of the respective near-isogenic non-Bt plants. However, the lignin content of these Bt plants, which was significantly less than that of Bt corn, was not significantly different from that of their near-isogenic non-Bt counterparts, although it was consistently higher. The Cry1Ab protein had no consistent effects on organisms (earthworms, nematodes, protozoa, bacteria, fungi) in soil or in vitro. The Cry1Ab protein was not taken up from soil by non-Bt corn, carrot, radish, or turnip grown in soil in which Bt corn had been grown or into which biomass of Bt corn had been incorporated.