The ubiquitin-proteasome system plays a central role in the degradation of intracellular proteins and is often required for efficient virus infection. Homologues of ubiquitin are found in all group I nucleopolyhedroviruses (NPVs), but their roles in NPV infection are still unclear. This study found that the specific proteasome inhibitor MG-132 markedly reduced budded virus (BV) production and polyhedrin expression in Bombyx mori NPV (BmNPV)-infected BmN-4 cells. Western blot analysis revealed that treatment of cells with MG-132 resulted in delayed and/or dysregulated viral gene product expression. Application of MG-132 significantly reduced BV production when applied up to 12 h post-infection (p.i.), whereas suppression of polyhedrin expression was almost abolished when applied after 6 h p.i. These results suggested that proteosomal degradation of viral and/or host proteins is required at an early stage of infection for efficient polyhedrin expression. To examine further the possible roles of ubiquitin signalling in BmNPV infection, the baculoviral ubiquitin gene (v-ubi) was deleted from the BmNPV genome. Deletion of v-ubi affected neither BV production nor polyhedrin expression. Furthermore, Western blots also showed that v-UBI was not required for degradation of IE2, which is known as a target viral protein of the ubiquitin-proteasome system.