It is important to resolve the structure of Bombyx mori silk fibroin before spinning (silk I) and after spinning (silk II), and the mechanism of the structural transition during fiber formation in developing new silk-like fiber. The silk I structure has been recently resolved by (13)C solid-state NMR as a "repeated beta-turn type II structure." Here, we used (13)C solid-state NMR to clarify the heterogeneous structure of the natural fiber from Bombyx mori silk fibroin in the silk II form. Interestingly, the (13)C CP/MAS NMR revealed a broad and asymmetric peak for the Ala Cbeta carbon. The relative proportions of the various heterogeneous components were determined from their relative peak intensities after line shape deconvolution. Namely, for 56% crystalline fraction (mainly repeated Ala-Gly-Ser-Gly-Ala-Gly sequences), 18% distorted beta-turn, 13% beta-sheet (parallel Ala residues), and 25% beta-sheet (alternating Ala residues). The remaining fraction of 44% amorphous Tyr-rich region, 22% in both distorted beta-turn and distorted beta-sheet. Such a heterogeneous structure including distorted beta-turn can be observed for the peptides (AG)(n) (n > 9 ). The structural change from silk I to silk II occurs exclusively for the sequence (Ala-Gly-Ser-Gly-Ala-Gly)(n) in B. mori silk fibroin. The generation of the heterogeneous structure can be studied by change in the Ala Cbeta peak of (13)C CP/MAS NMR spectra of the silk fibroin samples with different stretching ratios.