Among the three main components of cocoon sericin of the silkworm Bombyx mori, sericin A, the sericin secreted at the anterior portion of the middle silk gland, showed a different property compared to the two other components. After wetting the cocoon, sericin M and P decreased their solubility in hot water, and the decrease was enhanced by heat. On the other hand, sericin A apparently did not decrease its solubility after wetting and was extracted from cocoons even after being steamed. The FT-IR spectra of purified sericin films showed that sericin M could form a higher content of β-sheet structure than sericin A. These results indicate that the decrease in solubility of sericins M and P is due to β-sheet formation, which was considered to result from the serine-rich sequence composed of multiple 38-amino acid repeats.