This study is aimed to demonstrate histologically secretory materials of the rectal sac and also determine total proteolytic enzyme activity of sac’s fluid of silkworm, Bombyx mori considered to be responsible for adult eclosion from its cocoon. The rectal sacs of pupae were fixed with Bouin’s solution. The sections taken from the sac were stained by Paraldehyde Fuchsin and Periodic acid-Schiff dyes. Total protein concentrations and proteolytic activities of sac contents were determined by Bradford method at 595nm and azocasein substrate at 420nm, respectively. The secretions -proteinic and lipoid- were determined by different stains. As the sac grew up gradually, these two secretory materials were separated into darker and lighter phases. Protein concentration existed in the darker phase of secretion was higher than that in the lighter one. The total proteolytic activity measurements of the lighter phase were higher than the darker one, in both sexes. It was found that the activity of lighter phase in male is higher than that of female on the 10th day. This result supports that male moths escape earlier from cocoon than the female moths. Consequently, proteinic secretion synthesized by some cells of rectal sac could play a role in emerging from their cocoons of moths.