The open reading frame 7a of the Bombyx mori nucleopolyhedrovirus (BmNPV orf7a, Bm7a) encodes a predicted polypeptide consisting of 53 amino acid residues. While this polypeptide's homologues were found in certain baculoviruses, its function has not yet been studied. A protein structural assay revealed that the polypeptide expressed from this gene contained a predicted signal peptide at its N-terminus. To investigate the expression of Bm7a, 3'rapid amplification of cDNA ends was used to detect its transcript. A series of recombinant viruses were also constructed to analyze the cellular localization of the Bm7a products. The results indicated that the Bm7a gene is co-transcribed with bv/odv-e26 and orf9. The function of BM7a was studied by using fused enhanced green fluorescence protein (eGFP) as a visual marker to trace the localization of the protein during infection. Fluorescence was observed in the cellular membrane of the infected cells by confocal microscopy. Western blot analysis using an eGFP-antibody also showed that the BM7a-eGFP fusion protein was present in the budded virions (BVs). These data implied that BM7a is a component of the BV. In this work, we identified the function of the Bm7a gene and suggested that it may play a role in BV packaging.