Viruses employ host translational machinery to synthesize their own proteins while negatively controling host protein translation. Endoparasitoid wasp (Cotesia plutellae) parasitizes young larvae of the diamondback moth (Plutella xylostella) larvae and possesses at least 27 genome segments. Two viral genes (CpBV15α and CpBV15 β) were obtained from cDNA library of the parasitized larvae showing no homology with any known polydnaviral genes. The parasitized larvae did not produce storage protein 1 (SP1) among at least three polypeptides (SP1, SP2, and SP3) at the stage of polydnaviral CpBV15β synthesis. When CpBV15β protein, which was expressed in Sf9 cells, was incubated with fat body isolated from nonparasitized larvae, SP1 synthesis was markedly inhibited. In vitro translation of mRNAs from nonparasitized larvae using rabbit reticulocyte lysate with CpBV15β significantly resulted in inhibition of SP1 synthesis, suggesting a negative role of CpBV15β in host protein synthesis.