Abstract: The contact toxicity of sarisan against 5th instar larvae of Mythimna separate Walker was tested under the laboratory conditions with the method of topical application, their toxicity symptoms were observed, and its effects on the in vitro activities of AChE and ATPase and the content of ACh in heads or midguts of the larvae in different stages of toxicity were assayed. The results showed that the contact toxicity LD₅₀ value of sarisan against the 5th instar larvae of M. separate in 12 h post treatment was 106.93 μg/larva. In vivo toxicity symptoms were characterized by a definite sequence of events, starting with excitation, convulsion, paralysis, and death. Hollow duodenum was found in M. separata.AChE and Na+， K+-ATPase in M. separate larvae were strongly inhibited by sarisan, and a higher ACh content was caused  by sarisan. During excitation, convulsion and paralysis of treated larvae, the inhibition rates of AChE were 21.64%, 29.75% and 35.22%, respectively, while the inhibition rates of Na+，K+-ATPase were 33.29%, 44.45% and 23.98%, respectively.  

Key words: Mythimna separata；sarisan, Na+, K+-ATPase；AChE；ACh；enzyme activity