Abstract: Toxicity of 4 kinds of insecticidal crystal protein (ICP) from Bacillus thuringiensis (Bt) to Cry1Ac-resistant (R) and the homologous susceptible (S) strains of cotton bollworm, Helicoverpa armigera Hübner was evaluated. The sequence of toxicity of the 4 Bt ICP is as follows: Cry1Ac>Cry1Ac+1C>Cry2A>Cry1C to S strain, and Cry1Ac+1C>Cry1Ac>Cry2A>Cry1C to R strain. Significant synergism was found while appling Cry1c and Cry1c together to the R strain. At 1∶1 and 1∶2 proportions of the synergist and Cry1Ac notable synergism was observed. The synergism after 5 days treatment was higher than that after 14 days treatment, and was higher in the R strain than in the S strain.