The ichneumonid Diadromus collaris (Gravenhorst) (Hymenoptera: Ichneumonidae) is a major solitary, pupal endoparasitoid of the diamondback moth, Plutella xylostella (Linnaeus) (Lepidoptera: Plutellidae). Experiments to examine parasitism of the host pupae of different ages by the parasitoid were conducted in the laboratory. Diadromus collaris preferred host pupae that were in the first half of their pupal development. Survival from larva to adult, and size and parasitizing capacity of the resultant female adults decreased dramatically as host pupal age increased. When ovipositions were made into host pupae that were in the last quarter of their development, all parasitoids died before adult emergence. The performance of D. collaris, as affected by host pupal age, agrees with the simple diet theory that predicts female wasps should select hosts of higher nutritional quality for oviposition.