A novel cypovirus, assigned CoCPV, was isolated from natural populations of the western spruce budworm, Choristoneura occidentalis. The complete nucleotide sequences of genomic segments S2–S5 and S7–S10 were determined. Each segment contained a single open reading frame. Conserved motifs 5′ (AGUUU……UUUGUGC) 3′ were found at the ends of each segment. Analysis of S2, which encoded a putative RNA-dependent RNA polymerase protein, confirmed CoCPV belonged to the genus Cypovirus within the family Reoviridae. Further phylogenetic analysis using S10 (the polyhedrin gene) aligned this virus with species type-16, closely related to a cypovirus isolated from C. fumiferana.