Bombyx mori cytoplasmic polyhedrosis virus (BmCPV) strain H produces many hexahedral polyhedra (inclusion bodies) in the cytoplasm of insect midgut epithelial cells. The mutant A strain, however, produces polyhedra in the nucleus. We determined cDNA sequences of the polyhedrin genes, the smallest of the 10 genome segments, of these two strains. The polyhedrin genes of the H and A strains were 944 bp long, and encoded polypeptides of 248 amino acids (Mr 28,500) and 252 amino acids (Mr 29,000), respectively. The extra four amino acid residues at the carboxy terminus of the strain A polyhedrin (Arg-Leu-Leu-Val) were the result of a single nucleotide substitution at an opal stop codon (TGA----CGA). A further amino acid substitution of the histidine residue at position 101 (His----Tyr) was seen. The carboxy-terminal extension revealed a considerable similarity to the consensus amino acid sequence of the DNA-binding domain of many DNA-binding proteins. We discuss the relationship between the intracellular localization of polyhedrins and mutations in their amino acid sequences.