The insect glutathione S-transferases (GSTs) play an important role in the detoxification of xenobiotic compounds and are related to insecticides resistance. The full-length cDNA sequences encoding the sigma class GST protein (CsGSTsigma) was cloned from the Asiatic rice borer, Chilo suppressalis (Walker), one of the most important rice pests in Asia. The comparison of amino acid sequences showed that CsGSTsigma is highly similar to the sigma GST isolated from the domestic silkworm, Bombyx mori (L.). A homology model of CsGSTsigma was constructed and its binding environment for GSH is identical to that in the equivalent site of sigma GST from the fruit fly, Drosophila melanogaster Meigen. The developmental changes of the relative mRNA expression levels of CsGSTsigma were examined in Asiatic rice borer, and the highest expression level of this gene is in adult followed by the third-instar larvae stage. Furthermore, one gram-positive bacterium and two chemical insecticides were found to be able to induce the increasing expression of CsGSTsigma, suggesting that CsGSTsigma might work as an antioxidant enzyme to against the negative effects caused by both pathogens and xenobiotics.