Abstract: In 2004, the susceptibility to triazophos of striped stem borer (SSB), Chilo suppressalis (Walker) populations collected from Hangzhou district (Fuyang and Yuhang) and Jiaxin district (Pinghu, Xiucheng and Xiuzhou) of Zhejiang Province as well as pink stem borer (PSB), Sesamia inferens (Walker) population collected from Fuyang were assayed, respectively. Meanwhile, the activities of 4 enzymes related to the insecticide resistance including mixed function oxidase, glutathione S-transferase, esterase and acetylcholinesterase were determined. By comparison with a susceptible strain, there was a significant increase of 21.1- to 218.8- folds in LD50 value to triazophos in all SSB populations tested. SSB from Fuyang and Pinghu performed less sensitive to triazophos than that from Xiuzhou, Xiucheng and Yuhang. The results showed that the resistance mechanism to triazophos in SSB tested was related to increase of mixed function oxidase activity and insensitivity of acetylcholinesterase. PSB was more susceptible to triazophos than SSB from the same location, Fuyang, however, its resistance mechanism to triazophos need to be further studied.