The amorphous-region peptides obtained by chymotryptic digestion of large subunit of posterior silk gland fibroin were fractionated and characterized. The chymotryptic peptides were separated into four groups, S-I, II, III, and IV, by gel filtration on Sephadex G-15 and these four groups occupied about 32, 4, 46, and 17%, respectively, of the total peptides recovered. Each group (S-I-IN) was further fractionated on DEAE-Sephadex A-25 or Dowex-1 column chromatography and proved to be composed of peptide groups having molecular sizes of about 25-35, 8-15, 8-10 and 3-6 amino acid residues, respectively. The amino acid composition of the S-I peptides, a newly found group, was almost the same with that of the crystalline-region peptides of fibroin, but the molecular size of the former was about a half of the latter. S-III was composed of two main octapeptides, which were sequenced to be Gly-Ala-Gly-Ala-Gly-Ala-Gly-Tyr and Gly-Ala-Gly-Val-Gly-Ala-Gly-Tyr.