To clone the Bombyx xanthine dehydrogenase (XDH) gene as a dominant marker for silkworm transgenesis, we performed nested reverse transcriptase-polymerase chain reaction (RT-PCR) using embryonic mRNA and primers designed from the conserved region of Drosophila and rat XDH genes. Sequencing of amplified 180 bp fragments showed that two different sequences were present in the fragments. Since both possessed striking similarity to XDH genes of other organisms, we considered these to be portions of silkworm XDH genes and designated them BmXDH1 and BmXDH2. Subsequently we cloned separately the entire region of the two cDNAs by PCR using phage DNA of an embryonic cDNA library and sequenced them. The two cDNAs were around 4 kb in size and possessed complete open reading frames. The deduced amino acid sequences of the two BmXDHs were very similar to each other and to those of other organisms. The expression pattern of wild-type larvae basically followed the tissue specificity of the enzyme and no significant difference was observed between the two XDH genes. The expression of both genes was detected in the XDH-deficient mutants, oq and og, but non-synonymous substitutions were specifically detected in the BmXDH1 of the oq mutant. In addition, a length polymorphism of the second intron of the BmXDH1 co-segregated with the oq translucent phenotype, suggesting that deficiency in BmXDH is the cause of the oq translucent phenotype.