Abstract Euplectrus separatae is a gregarious ectoparasitoid that parasitizes Pseudaletia separata during its third to sixth (last) instars. The eggs of the parasitoid are fixed on the integument of the host dorsolaterally with a hard substance like a piling driven into the integument by the female wasp at the time of oviposition. The first instar of the parasitoid, which hatches three days after oviposition is nourished by ingesting the hemolymph of the host, and ecdyses to the second stadium six days after oviposition. Many hemocytes and epidermal cells were found assembled under the piling and places where a parasitoid had attached its mouth, suggesting that the host had repaired the integument destroyed by the parasitoid. Botryoidal tissue, which stained well with hematoxylin, began to develop from four days after oviposition and became gradually larger with development. Botryoidal tissue appears to function as a secretory organ for thread and a storage organ for nutrients. Seven days after oviposition, the parasitoid larvae migrate down from the dorsal surface to the ventral side of the host. Just before descending they ecdyse to the third stadium and kill the host during their migration. If all parasitoid larvae were removed artificially from the host before they migrate, the host did not die. However, removing the parasitoids after they had started to migrate did not prevent the death of the host. Transmission electron microscopic (TEM) observation of salivary glands of a parasitoid larva before migrating revealed that the salivary gland was composed of cells that were rich in rough surfaced endoplasmic reticulum (rough-ER) with many ribosomes and cells that were filled with a lot of vacuoles just before their collapse. After moving from the host body, the parasitoid larvae doubled in weight by ingesting the tissue of the host and then spun a cocoon. Almost all host tissues were consumed for growth of the parasitoid, like an idiobiont parasitoid.