Diadegma semiclausum (Hellén) (Hymenoptera: Ichneumonidae) is one of the most important biological control agents of the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae). Each immature stage of D. semiclausum was studied by dissecting parasitized hosts in the laboratory at 22 ± 1°C and 70–75% RH. Results show that the egg is hymenopteriform, lacking appendages. The larvae seem to be hypermetamorphic, with four instars. The pupal period has two distinct phases: 1) phase I, larval-like with remarkable groups of urate cells; and 2) phase II, exarate pupal appearance with a pigmentation sequence of eyes, ocelli, mandibles, thorax, abdomen, and antennae. All stages and sequences are illustrated with detailed photomicrographs.