The complete nucleotide sequences of genomic segments S5 to S10 from Operophtera brumata cypovirus 18 (OpbuCPV18), and the complete nucleotide sequences of genomic segments S2, S5, S9 and S10 from Operophtera brumata cypovirus 19 (OpbuCPV19) have been determined. Each genome segment contained a single open reading frame (ORF). Conserved motifs 5′ (AGUAAA....GUUAGCU) 3′ were found at the ends of each segment of OpbuCPV18, whilst conserved motifs 5′ (AACAAA....UUUGC) 3′ were found at each segment terminus of OpbuCPV19. The putative proteins were compared with those of other members of the Reoviridae family. Phylogenetic analysis using the polyhedrin gene (S10) indicated that OpbuCPV18 was most closely related to Dendrolimus punctatus cypovirus 1, whilst OpbuCPV19 was most closely related to Trichoplusia ni cypovirus 15. In addition, analysis of S2, which encoded a putative RNA-dependant RNA polymerase gene, confirmed OpbuCPV19 belonged to the genus Cypovirus. Following the expression of the ORF from OpbuCPV19 S10, using a baculovirus expression vector, occlusion bodies were observed in insect cell culture. This demonstrated that segment 10 coded for the polyhedrin gene, capable of forming a polyhedral crystalline matrix.