Spodoptera litura nucleopolyhedrovirus (SpltMNPV) vp39 (Spltvp39) is one of the 37 core genes identified in all sequenced baculovirus genomes. The open reading frame of Spltvp39 is 909 bp in length, which encodes 302 amino acids with predicted molecular weight of 33.9 kDa. Homology analysis indicated that SpltVP39 protein has 20-67% amino acid identity with those of other known baculoviruses. Amino acid sequence alignment of SpltVP39 homologs showed 6 highly conserved cysteine residues. In this study, the rabbit polyclonal SpltVP39 antiserum was prepared. Spltvp39 transcription in SpltMNPV-infected SpLi-221 cells was first detected at 12 h post-infection (p.i.), and reached the maximal level at 96 h p.i.. The transcriptional start sites of mRNA were mapped within 3 consensus baculovirus late promoter motifs ATAAG. Time course analysis of SpltVP39 expression in SpltMNPV-infected cells showed that SpltVP39 was expressed from 24 through 96 h p.i. as a 33.9 kDa protein. Western blotting indicated that this protein appeared in the nucleocapsids of both occlusion-derived virions and budded virions. These results demonstrate that the Spltvp39 should be classified as a baculovirus late gene similar to most other baculovirus late structural genes.