The tumor suppressor protein p53 serves as a crucial mediator of apoptosis induction and is negatively regulated by various upstream cellular factors through post-translational modifications, including ubiquitination. Here, we identified and characterized the coding region of Bombyx mori homologs of bonus, mdm2, sce, and synoviolin with E3 ubiquitin ligase activity, and rad6 with E2 ubiquitin conjugating enzyme activity, which negatively regulate p53 levels and contribute to p53-mediated apoptosis induction in Drosophila melanogaster and/or mammals. Among the characterized B. mori homologs, Bm-p53 (B. mori homolog of p53) responded to BmMdm2 alone. RNAi-mediated knockdown of bm-mdm2 expression caused increased cellular Bm-p53 levels, whereas transient overexpression of Bm-Mdm2 resulted in reduced cellular Bm-p53 levels, indicating that BmMdm2 functions as a negative regulator of Bm-p53 in B. mori cells. Despite considerable increase in Bm-p53 levels after RNAi-mediated knockdown of bm-mdm2 expression, apoptosis induction or caspase activation was undetectable in B. mori cells under the experimental conditions used. In contrast, transient co-overexpression of Bm-p53 and Bm-Mdm2 attenuated Bm-p53-induced apoptosis and effector caspase activity of B. mori cells without an appreciable reduction in Bm-p53 levels that are expressed by corresponding transfected plasmids. These results indicate that Bm-Mdm2 is the prime negative regulator for Bm-p53 involved in apoptosis induction of B. mori cells.