A wasp, Cotesia plutellae, parasitizes the diamondback moth, Plutella xylostella, and interrupts host physiology for wasp survival and development. Identification of parasitism-specific factors would be helpful to understand the host–parasitoid interaction. This study focused on identification of a 15-kDa protein found only in plasma of the parasitized P. xylostella. Degenerate primers were designed after N-terminal amino acid sequencing of the parasitism-specific protein and used to clone the corresponding gene from the parasitized P. xylostella by a nested reverse transcriptase-polymerase chain reaction (RT-PCR). Two homologous genes were cloned and identified as “CpBV15α” and “CpBV15β,” respectively, due to the identical size (158 amino acid residues) of the predicted open reading frames, in which they shared amino acid sequences in both terminal regions, but varied in internal sequences. Southern hybridization analysis indicated that both genes were located on C. plutellae bracovirus genome. Real-time quantitative RT-PCR revealed that both genes were mostly expressed at the late parasitization period, which was further confirmed by an immunoblotting assay using CpBV15 antibody. A recombinant CpBV15 protein was produced from Sf9 cells via a baculovirus expression system. The purified CpBV15 protein could enter hemocytes of P. xylostella and were localized in the cytosol. Along with the sequence similarities of CpBV15s with eukaryotic initiation factors, their putative biological role has been discussed in terms of the host translation inhibitory factor. Arch. Insect Biochem. Physiol. 67:157–171, 2008. © 2008 Wiley-Liss, Inc.