1. We have cloned the cDNA encoding the vitellin (Vtn)-degrading protease (30 k Vtn protease and 24 k Vtn protease) of the silkworm, Bombyx mori, and determined the primary structure by sequencing the cDNA along with mRNA. 2. The deduced amino acid sequence comprised 264 amino acid residues and had high homology to the trypsin-like proteases of vertebrates and invertebrates. 3. Northern blot analysis using the cDNA as a probe revealed that the transcription of the Vtn protease gene occurred at the restricted stage of embryogenesis when the protease activity appeared. 4. The in vitro translation experiment demonstrated that a 32 kDa polypeptide was the primary translation product and the translation activity changed according to transcriptional activity. 5. By Western blotting using the antiserum against each Vtn protease, two enzymes were shown to share the common antigenicity, and the titer of both enzyme proteins changed closely related with activity of proteases. 6. These results led us to conclude that the biosynthesis of Vtn protease is regulated at the level of transcription.