HPLC was used to investigate whether spinosad applied to parasitized host larvae could reach parasitoid larvae developing inside the hosts. Plutella xylostella larvae of a strain highly resistant to spinosad were used as hosts to rear the parasitoid Cotesia plutellae. When the parasitoids had developed to the stage of early instar larva I the host larvae were treated with 50 mg/L spinosad on the terga by topical application and then fed with cabbage leaves treated with 50 mg/L spinosad to allow the host larvae and the parasitoid larvae to develop. Three days after treatment, the parasitoid larvae inside the host were collected by dissecting the host larvae. The body fluid of P. xylostella and the bodies of parasitoid larvae were then analyzed with HPLC. Both of the active components, spinosyn A and spinosyn D, of spinosad were detected in the body fluid of host larvae and in the bodies of parasitoid larvae, and the residue concentrations were 2.79 mg/L and 0.94 mg/L in the host and the parasitoid respectively. The results demonstrate that spinosad applied to the host larvae can in turn reach inside the parasitoid larvae through penetration of cuticle and/or feeding of host hemolymph by the parasitoid larvae, and subsequently acts on the parasitoid larvae. none