The Cry3Bb1 protein, insecticidal to the corn rootworm complex (Diabrotica spp.), of Bacillus thuringiensis (Bt) subsp. kumamotoensis was released in root exudates of transgenic Bt corn (event MON863) in sterile hydroponic culture (7.5±1.12 ng ml^-1 after 28 d of growth) and in nonsterile soil throughout growth of the plants (2.2±0.62 ng g^-1 after 63 d of growth). Kitchawan soil, which contains predominantly kaolinite (K) but not montmorillonite (M), was amended to 3 or 6% (vol vol^-1) with K (3K and 6K soils) or M (3M and 6M soils) and with 1, 3, 5, or 10% (wt wt^-1) of ground biomass of Bt corn expressing the Cry3Bb1 protein and incubated at 25±2^oC at the –33-kPa water tension for 60 d. Soils were analyzed for the presence of the protein every 7 to 10 d with a Western blot test (ImmunoStrip) and verified by enzyme-linked immunosorbent assay (ELISA). Persistence of the protein varied with the type and amount of clay mineral and the pH of the soils and increased as the concentration of K was increased but decreased as the concentration of M was increased. Persistence decreased when the pH of the K-amended soils was increased from ca. 5 to ca. 7 with CaCO₃:  the protein was not detected after 14 and 21 d in the pH-adjusted 3K and 6K soils, respectively, whereas it was detected after 40 d in the 3K and 6K soils not adjusted to pH 7. The protein was detected for only 21 d in the 3M soil and for 14 d in the 6M soil, which were not adjusted in pH. These results indicate that the Cry3Bb1 protein does not persist or accumulate in soil and is degraded rapidly.