Braconid wasps, Cotesia plutellae (Kurdjumov), were collected from parasitized host larvae of diamondback moth, Plutella xylostella (L.) in Korea. Virus particles were found in the oviduct lumen of C. plutellae females. Multiple nucleocapsids with [approximately]30-nm diameter and variable length (30-80 nm) were surrounded with a single unit membrane envelope. The parasitization of C. plutellae completely inhibited pupal metamorphosis. The parasitized larvae showed significant decrease in feeding activity and total hemolymph proteins, especially as larval storage proteins. They also showed a significant decrease in immune capacity as evidenced by reduced ability to form hemocyte nodules and reduced phenoloxidase and lysozyme activity. Here, we show that C. plutellae has an endosymbiotic virus like other reported species in Microgastrinae, and suggest that it causes host developmental arrest and immune-depression at parasitization. [copyright] 2004 Elsevier Inc. All rights reserved. Biochemistry; Nutrition; Circulatory system; Life cycle and development; Development; Associations; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Immunology and repair mechanisms Plutella xylostella (Plutellidae); Proteins; Feeding rate; Haemolymph; Haemolymph proteins; Metamorphosis; Associations; Virus; Hymenopteran parasites; Cotesia plutellae; Host physiological changes; Immune like reactions Cotesia plutellae (Braconidae); Lepidopteran hosts; Plutella xylostella; Host physiological changes none