The ultrastructure of a-chitin microfibril produced by marine alga Phaeocystis was investigated by FT-IR spectroscopy, X-ray diffraction and electron microscopy. The average size of the microfibril was 17.1 +/- 1.8 mu m in length and 39.8 +/- 8.8 nm in width. The FT-IR spectrum shows typical alpha-chitin pattern, and each band was sharper than crustacean chitin's, indicating higher crystallinity of the Phaeocystis chitin. The X-ray diffraction gave crystallite size more than twice of crustacean tendon's. The fiber diffraction pattern is consistent with previous studies with two-chains orthorhombic unit cell (Minke and Blackwell, 1978; Sikorski et al., 2009), and refined unit cell dimensions are a = 4.742 angstrom, b = 18.871 angstrom, c= 10.338 angstrom, High-resolution electron microscopy of ultrathin sections gave the cross-sectional shape of microfibril as hexagon. The lattice images of (0 2 0) plane (d = 0.94 nm) were frequently observed extending the entire cross-section of microfibril, indicating its single crystalline nature. These results allowed construction of a molecular packing model for alpha-chitin crystal. (C) 2010 Elsevier Inc. All rights reserved.