Bombyx mori nucleopolyhedrovirus (BmNPV) ORF109 (Bm109) is a gene present in many lepidopteran NPVs, but its function is unknown. In this study, Bm109 was characterized. The transcript of Bm109 was detected at 12-72 h postinfection (p.i.). Polyclonal antiserum raised to a His-BM109 fusion protein recognized BM109 in infected cell lysates from 24 to 72 h p.i., suggesting that Bm109 is a late gene. Localization of the BM109 in BV and ODV by western analyses demonstrated that BM109 was proteins of ODV.