Parasitized by the endoparasitoid Cotesia plutellae (Hymenoptera: Braconidae), the feeding, development and nutrition metabolism of the host Plutella xylostella (Lepidoptera: Plutellidae) were changed dramatically as the immature parasitoid developed inside. These changes were brought about by the impact of various parasitoid-associated factors. Teratocyte as one factor plays an important role. By comparing the changes of proteins in parasitized host and un-parasitized host haemolymphs and the culture mediums of 2nd instar parasitoid larvae using these two kinds of haemolymph in vitro, we found that the protein concentration in the hemolymph of the parasitized host was only slightly (not significantly) lower than that of the unparasitized host, but that of the culture medium using unparasitized host hemolymph was significantly lower than that of parasitized host, suggesting that the teratocytes have the ability to secret proteins. The body weight of parasitized host was g! reater than that of un-parasitized host while the reverse was true for the weight of fat body in the late stage of parasitization. Microscopic observation revealed that the moniliform fat body was broken into granules, which was induced by teratocytes attached the fat body tissue; correspondingly, the soluble protein and lipid concentrations of fat body decreased rapidly, much lower compared to those of unparasitized host. At the same time parasitoid larva grew rapidly, its midgut became mature and absorbed a large amount of the host nutrients, the esterase activity in the midgut of parasitoid larvae increased accordingly and the amount of total lipid in the parasitoid larva increased up to its highest level. In conclusion, in the late stage of parasitization, the parasitoid larva, with the help of teratocytes, utilized up most of the host nutrients for its own development and growth. BR Parasites diseases and disorders. Parasites. Insect parasites. Hosts. Insect hosts. Plutella xylostella (Plutellidae): Host; Cotesia plutellae (Braconidae): Parasite. Cotesia plutellae: Lepidopteran hosts, Plutella xylostella, Parasite regulation and utilization of host nutrients; Plutella xylostella: Hymenopteran parasites, Cotesia plutellae, Parasite regulation and utilization of host nutrients. ST Insecta, Lepidoptera, Glossata, Heteroneura, Yponomeutoidea, Plutellidae; Insecta, Hymenoptera, Apocrita, Terebrantia, Ichneumonoidea, Braconidae. Arthropods; Hymenopterans; Insects; Invertebrates; Lepidopterans. none