A model system of transgenic cotton that produces Bacillus thuringiensis var. kurstaki delta -endotoxin (Btk toxin) was used to evaluate the persistence of transgenic pesticides in a sandy loam soil from California, USA. Purified Btk toxin or transgenic cotton leaves containing Btk toxin were added to soil in five different microcosm experiments in concentrations ranging from 1 to 1600 ng Btk toxin/g soil. The concentration of the extractable Btk toxin was measured for up to 140 days. An initial rapid decline in extractable toxin concentration in the first 14 days, followed by a slower decline, was observed in four of the five experiments. At the end of the experiments, Btk toxin from transgenic plant tissue was undetectable (<0.1% of starting concentration) in two of the microcosm experiments and at 3, 16, and 35% of the original amounts in the other experiments. In addition, experiments using gamma -irradiated sterilized soil showed that the observed decline in extractable toxin concentration was due largely to biotic degradation rather than to physical adsorption by the soil.