The nucleotide sequence of the Helicoverpa armigera single-nucleocapsid nucleopolyhedrovirus (HaSNPV) DNA genome was determined and analysed. The circular genome encompasses 131403 bp, has a G+C content of 39·1 mol% and contains five homologous regions with a unique pattern of repeats. Computer-assisted analysis revealed 135 putative ORFs of 150 nt or larger; 100 ORFs have homologues in Autographa californica multicapsid NPV (AcMNPV) and a further 15 ORFs have homologues in other baculoviruses such as Lymantria dispar MNPV (LdMNPV), Spodoptera exigua MNPV (SeMNPV) and Xestia c-nigrum granulovirus (XcGV). Twenty ORFs are unique to HaSNPV without homologues in GenBank. Among the six previously sequenced baculoviruses, AcMNPV, Bombyx mori NPV (BmNPV), Orgyia pseudotsugata MNPV (OpMNPV), SeMNPV, LdMNPV and XcGV, 65 ORFs are conserved and hence are considered as core baculovirus genes. The mean overall amino acid identity of HaSNPV ORFs was the highest with SeMNPV and LdMNPV homologues. Other than three ‘baculovirus repeat ORFs’ (bro) and two ‘inhibitor of apoptosis’ (iap) genes, no duplicated ORFs were found. A putative ORF showing similarity to poly(ADP-ribose) glycohydrolases (parg) was newly identified. The HaSNPV genome lacks a homologue of the major budded virus (BV) glycoprotein gene, gp64, of AcMNPV, BmNPV and OpMNPV. Instead, a homologue of SeMNPV ORF8, encoding the major BV envelope protein, has been identified. GeneParityPlot analysis suggests that HaSNPV, SeMNPV and LdMNPV (group II) have structural genomic features in common and are distinct from the group I NPVs and from the granuloviruses. Cluster alignment between group I and group II baculoviruses suggests that they have a common ancestor.