Inhibitor <SUB>K</SUB>B (I<SUB>K</SUB>B)-like gene has been found ill the genome of Cotesia plutellae bracovirus (CpBV), which is the obligatory symbiont of an endoparsitoid wasp, C. plutellae. The open reading frame of CpBV-I<SUB>K</SUB>B was 417 bp and encoded 138 amino acids. Four ankyrin repeat domains were found in CpBV-I<SUB>K</SUB>B, which shared high homology with other known polydnavirus I<SUB>K</SUB>Bs. Considering a presumptive cellular I<SUB>K</SUB>B based on Drosophila Cactus, CpBV-I<SUB>K</SUB>B exhibited a truncated structure with deletion of signal-receiving domains, which suggested its irreversible inhibitory role in NF<SUB>K</SUB>B signal transduction pathway of the parasitized host in response to the wasp parasitization. CpBV-I<SUB>K</SUB>B was expressed only in the parasitized diamondback moth, Plutella xylostella. Its expression was estimated by quantitative RT-PCR during parasitization period, showing a constitutive expression pattern from the first day of parasitization. An indirect functional analysis of CpBV-I<SUB>K</SUB>B was conducted and suggested a hypothesis of host antivirus inhibition.