Cotesia vestalis (Haliday) (Hymenoptera: Braconidae) is one of the most important biological control agents of the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae). Immature development of C. vestalis was studied by dissecting parasitized hosts in the laboratory at 25 ± 1°C and 60–65% RH. Results showed that the egg matured in 2 d. Larvae seemed to have three instars; the first two larval instars molted inside the host, and the third instar emerged from the host to spin a cocoon. The first, second, and third instars lasted 2, 5, and 1 d, respectively. The first instar is transparent and caudate-mandibulate with a large head, distinct mandibles, and a caudal projection. The second instar is semitransparent with an anal vesicle. The third instar larva is yellowish green and the body is slightly curved. The pupa develops for 5 d and is bright yellow with transparent antennae, wings, and legs. All stages were fully documented with microscopic photos.