Cotesia vestalis is an endoparasitoid of Plutella xylostella larvae and injects a polydnavirus (CvBV) into its host during oviposition. In this report we characterize the gene, CvBV3307, and its products. CvBV3307 is located on segment S33 of the CvBV genome, is 517 bp, and encodes a putative protein of 122 amino acids, including a serine-rich region. The expression pattern of CvBV3307 in parasitized larvae and the subcellular localization of CvBV3307 only in granulocytes indicated that it might be involved in early protection of parasitoid eggs from host cellular encapsulation and in manipulating the hormone titer and developmental rhythm of host larvae. Western blot analysis showed that the size of the immunoreactive protein (about 55 kDa) in parasitized hosts at 48 hours post parasitization (h p.p.) is much larger than the predicted molecular weight of 13.6 kDa, which suggests that CvBV3307 undergoes extensive post-translational modification in hosts.