Bmdsx is an orthologue of the sex-determining gene doublesex (dsx) and known to be sex-specifically expressed in various tissues of the silkworm, Bombyx mori. Its pre-mRNA is sex-specifically spliced and encodes female-specific or male-specific polypeptides. The open reading frame of Bmdsx consists of 5 exons, of which exons 3 and 4 are female-specific and its pre-mRNA was known to undergo default processing to generate the female-type mRNA. Previous reports have shown that the mechanism of splicing of the doublesex gene is different in Drosophila melanogaster and Bombyx mori. However, intron 4 is so long that it is difficult to identify the intronic cis-element(s) required for male-specific splicing of Bmdsx pre-mRNA using Bmdsx minigenes whose introns are shortened in various manners. As a first step toward discovery of the cis-element, the Bmdsx mini gene, which consisted of exon 1 and 5 and internally shortened introns 2 to 4, was constructed, and transgenic silkworms expressing this construct were generated. Bmdsx pre-mRNA transcribed derived from transgene was sex-specifically spliced. This result shows that the mini gene contained the information necessary for the correct regulation of alternative splicing.