Dow AgroSciences has modified cotton plants (Gossypium hirsutum L.) by inserting a gene from Bacillus thuringiensis Berliner subsp. aizawai that produces an insect-active, full-length Cry1F delta-endotoxin, and is planning to commercialize insect-resistant cotton lines expressing this protein. These cotton lines control tobacco budworm, Heliothis virescens (F.), along with other lepidopteran larvae. A laboratory study was conducted to better understand the degradation of Cry1F protein in a representative soil from the midsouth cotton-growing area of the United States. Bioassay results with tobacco budworm were used to measure the relative titer of the protein in the soil after a series of incubation periods. Based on the decrease in toxicity over time, the half-life of the microbial Cry1F delta-endotoxin was estimated to be less than 1 d under laboratory conditions, indicating a rapid decay rate in soil.