We examined development and host-stage selection by Cotesia plutellae, a larval parasitoid of the diamondback moth, Plutella xylostella. There were three larval instars in C. plutellae. Like in other solitary endoparasitoids, first instar larva had sclerotized madibles, which may function to destroy other larvae of conspecific or heterospecifics in the same host. Third instar larva tore out the side of host larvae and emerged. Then, they span the cocoons within 2-4 hours after larval emergence. Females usually laid a single egg in each host, though they superparasitized hosts of the second and third instars more often than the hosts of the first and fourth instars. Although females parasitized all four larval instars of P. xylostella, they preferred to oviposit in the hosts of the second and third instars than rather the hosts of the first and fourth instars. When 50 hosts of either the first, second, third or fourth instar were exposed to two females, the hosts of the second and third instars yielded more parasitoid cocoons than the hosts of the first and fourth instars. Developmental time from oviposition to adult emergence decreased with the increasing host instar at oviposition. Based on the results on acceptance, preference and suitability, the second and third instar larvae of P. xylotealla were suitable for parasitism by Cotesia plutellae. none