The ultrastructure of beta-chitin microfibrils from a centric diatom, Thalassiosira, and a tubeworm, Lamellibrachia, was studied using electron diffraction and high-resolution electron microscopy. Electron micro-diffraction diagrams corresponding to each projection of the beta-chitin crystals were obtained, and all the data support the structure model of anhydrous beta-chitin crystals proposed by X-ray diffraction experiments. From high-resolution electron microscopy on ultrathin sections, the cross-sectional shapes of the microfibrils from Thalassiosira and Lamellibrachia were observed as a rectangular and parallelogram, respectively. The lattice fringes corresponding to the (0 1 0) plane of anhydrous beta-chitin crystals were clearly observed in both cross-sections. Based on these observations, we have constructed a molecular packing model for beta-chitin microfibrils. (C) 2011 Elsevier Inc. All rights reserved.