Abstract: The main insecticidal toxin from Xenorhabdus nematophila HB310 was a kind of high molecular weight protein complex—toxin Ⅱ.The effects of the culture broth and the toxinⅡon the growth and several midgut enzymatic activities of the H.armigera larvae were investigated.The results indicated that the culture broth and the toxinⅡ not only affected the feeding food quantity but also disrupted the development of the H.armigera larvae (including larvae period,pupation percentage,pupa weight and emergence percentage).The growth of the 2nd instar larvae exposed to 6.5×108 cells/mL culture broth was almost entirely inhibited, and all the larvae did not pupated.The growth of the 2nd instar larvae exposed to 1.3×107 cells/mL culture broth was still markedly inhibited.The growth of the 4th instar larvae exposed to 51.9 μg/mL toxinⅡ was also inhibited remarkably.Short time exposure to the culture broth prolonged the development period of the 2nd and 4th instar larvae,but did not impact on the pupation percentage,pupa weight and emergence percentage of H.armigera.Toxin Ⅱ also significantly inhibited the primary enzymatic activities of the midgut of H.armigera larvae.