Abstract: Penetration and metabolism of celangulin V in the larvae of Mythimna separata and Agrotis ypsilon were studied using high performance liquid chromatography. The results showed that celangulin V can not penetrate into the haemolymph from the body wall or penetrate into the midgut from the haemolymph in both of larvae. Whereas, celangulin V with the same rate of penetration is easily penetrate into the haemolymph from the midgut in the case of both larvae. The metabolic rate of celangulin V in the larvae of A.ypsilon is greater than that in the larvae of M. separata and the half-life of celangulin V was 5.5 h and 13.1 h in the larvae of A.ypsilon and M. separata, respectively. The results indicate that the penetration was not involved in the selective toxicity of celangulin V, whereas, detoxification metabolism in the larvae of A.ypsilon and M. separata was one of the mechanisms of this selective toxicity.