ChaB, a putative regulator of ChaA in Escherichia coli (E. coli), and its homologues constitute a multigene family known to occur among bacteria, archaeabacteria and baculoviruses. The baculovirus Spodoptera exigua multicapsid nucleopolyhedrovirus contains two homologues of E. coli ChaB, open reading frames (ORFs) 100 (Se100) and 101 (Se101). RT-PCR and 5' RACE analyses indicated that transcription of both SeMNPV chaB genes occurs by 24 h postinfection and is initiated at a late consensus ATAAG motif. Immunoblot analysis showed that Se100 and Se101 were, respectively, expressed as 11 and 28kDa proteins in infected cells, and they were distributed in both cytoplasm and nucleus. Further analysis revealed that Se100 and Se101 are associated with nuclear structure and only present in the nucleocapsid of occlusion-derived virus. Additionally, column chromatography analysis showed that both proteins could weakly interact with nucleic acids. It was proposed that SeMNPV ChaB might function as DNA binding proteins.