Abstract:The levels of Bt toxic protein in six kinds of transgenic Bt cottons in different periods were detected using ELISA and the levels of the resistance of those cottons against cotton bollworm larvae of local four generations were tested using laboratory bioassay. The levels of Bt toxic protein in tender leaves of transgenic Bt cotton decreased gradually with the cotton growing up. The level of Bt toxic protein was highest in cotyledon period, reaching 1 210.03-1 733.15 ng/g. Compared with the cotyledon period, the reduced ranges of the Bt toxic protein in tender leaves of three-leaf period, seven-leaf period, squaring period, flowering and little boll period, boll opening period were 2.5%-96.0%. The resistance of those cottons to 4th generation larvae was lower, and only up to 19.0%-41.3%. The corrected mortality of the 4th generation larvae were merely only 12.0%-36.2%, lower resistance were indicated.