The microstructure and its crystallographic aspect of the shell of a limpet, Lottia kogamogai, have been investigated, as the first step to clarify the mechanism of shell formation in limpet. The shell consists of five distinct layers stacked along the shell thickness direction. Transmission electron microscopy (TEM) with the focused ion beam (FIB) sample preparation technique was primarily adopted, as well as scanning electron microscopy (SEM) with electron back-scattered diffraction (EBSD). The five layers were termed as M + 3, M + 2, M + 1, M, M - 1 from the outside to the inside in previous works, where M means myostracum. The outmost M + 3 layer consists of calcite with a "mosaic" structure; granular submicron sub-grains with small-angle grain boundaries often accompanying dislocation arrays. M + 2 layer consists of flat prismatic aragonite crystals with a leaf-like cross section, stacked obliquely to the shell surface. It looks that the prismatic crystals are surrounded by organic sheets, forming a compartment structure. M + 1 and M 1 layers adopt a crossed lamellar structure consisting of aragonite flat prisms with rectangular cross section. M layer has a prismatic structure of aragonite perpendicular to the shell surface and with irregular shaped cross sections. Distinct organic sheets were not observed between the crystals in M + 1, M and M - 1 layers. The (1 1 0) twins are common in all aragonite M + 2, M + 1, M and M - 1 layers, with the twin boundaries parallel to the prisms. These results for the microstructure of each layer should be considered in the discussion of the formation mechanism of the limpet shell structure. (C) 2010 Elsevier Inc. All rights reserved.