Using leaf or square feeding bioassay. the insect resistance of transgenic Bt cotton (Bollgard?Xinmian33B) leaves and squares to neonates of susceptible Helicoverpa armigera (Hubner) were determined in 2000. 2001 and 2003, respectively. The results showed that the insect resistance of the 33B to susceptible neonates decreased with increase of the leaf order on the main stem . The insect resistance of the 6th - 10th leaf on the main stem was very high (the mean mortality was 91.4 - 100 %, the same as follow)...