Larvae of a gregarious endoparasitoid, Cotesia kariyai (Watanabe), grew rapidly during the second stadium in the host. The fat body of a Pseudaletia host parasitized by C. kariyai was completely consumed by 10 d, just before larval emergence. It seemed hard to explain the growth of the second instar parasitoids and the rapid consumption of the fat body only by ingestion of hemolymph converted from the fat body or other organs of the host. Paraffin sections of the parasitized host revealed that many teratocytes were attached to the surface of the fat body in many sites and destroyed the fat body tissue locally. Zymography of proteins released from the teratocytes revealed that the teratocytes 4 to 9 days after parasitization showed collagenase activity (as a gelatinase). Further, 1st instar parasitoids which were transplanted together with teratocytes into unparasitized hosts preconditioned with C. kariyai polydnavirus (CkPDV) plus venom, grew normally to the 2nd stadium. Abnormal growth of parasitoid larvae was observed when parasitoid larvae were transplanted without teratocytes. These results suggest that the teratocytes attach to the outer sheath of the fat body, secrete an enzyme that makes a hole in the matrix of the fat body, thus allowing the second instar parasitoid to ingest the content of the fat body.