Cowpea trypsin inhibitors (CPTI) are small polypeptides of around 80 amino acids belonging to the Bowman-Birk type of double-headed serine protease inhibitors. It has been found that the gene encoding a CPTI can confer enhanced resistance to a wide range of pests in transgenic plants. However, the ultimate effectiveness of this method can only be determined by testing for the resistance in the field. After we transformed the gene of a CPTI into a major commercial tobacco cultivar, NC89, and obtained 2 transgenic plants highly resistant to H. assulta, therefore, we retested the resistance of the progenies of the 2 plants under field conditions. The field plot was divided into two blocks, A and B. In the A block, every tobacco plant was infested with four newly emerged larvaes of H. assulta 15 days after transplanting. In comparison with the non-transformed control, mortality of the insects feeded on the transgenic plants averaged 20% and the growth of the survived insects was remarkably inhibited 15 days after infestion. In the B block, no insecticide was applied (the larvaes reached fourth instar when removed by hand) and the insect-damage index of the offspring of two transgenic plants was 37.3 and 39.8 respectively, in contrast to about 57 of the control 30 days after transplanting. These results demonstrated that the expression of a CPTI gene in transgenic tobacco leads to high resistance to insects, and the resistance proves to be effective under field conditions. The engineered tobacco lines represent a significant approach to increased selectivity, specificity and efficiency in insect control.