pH of the liquid silk in the glandular lumen was estimated in situ by injecting the pH-sensitive dye, phenol red. In the 5th instar feeding larvae of Bombyx mori, the pH value of the lumen in the middle part of the middle silk gland (MSG-mid) was around 5.5. That of the posterior part of MSG (MSG-post) was around pH 6.5. That in the posterior silk gland (PSG) was pH 7-8. In the spinning larvae, pH at the MSG-mid was neutralized to >pH 6. These tendency was also confirmed in the silk gland of Samia cynthia ricini.