Abstract:The insecticidal toxin genes in Xenorhabdus nematophila BP can express toxin proteins that have high insecticidal activity to many Lepidoptera larvae. As a kind of candidate toxin gene for transgenic insect-resistant plants, it is very important to found out the relationships of these genes. Sub9.0, one subclone of a cosmid clone XnBP83 from Xenorhabdus nematophila BP which has all xpt insecticidal genes, was used for studying the function of xptA2 in X.nematophila because only xptA2 presented in this subclone. It was found that protein expressed by xptA2 had oral toxicity to Heliothis armigera but the full oral toxicity of XnBP83 could not be restored when physical mixing of toxins expressed by the 3 genes, xptB1, xptC1 and xptA2. The full oral toxicity of XnBP83 could not be restored when xptA2 was transformed into XnBP76 or XnBP203. Two varied regions were found in the deduced amino acid of xptA2 of X.nematophila BP comparing with that in PMFI296. BLAST results showed that the two regions had homology with XptA1 in X.nematophila PMF1296 and TcdA in P.luminescens W14 which were thought to be associated with oral toxicity to insects.