This paper aims to elucidate the structure and characteristic mineralization of the gastric mill cuticle in the crayfish Procambarus clarkii (Girard) using light and transmission electron microscopy. Structure and mineralization of the lateral teeth of the gastric mill in the intermolt stage are documented. The lateral tooth is composed of two types: incisor-like teeth and molar-like teeth. The tooth structure is divided into three regions: “large-cusp region” is the protuberance region in the incisor-like teeth, “small-cusp region” is the protuberance region in the molar-like teeth, and “marginal region” is the surrounding region. The cuticle is divided into epicuticle, exocuticle, endocuticle, and membranous layer. The cuticle of the large-cusp region lacks epicuticle and endocuticle, while the cuticle of the small-cusp region lacks the epicuticle. The cuticle of the marginal region is composed of epicuticle, exocuticle, endocuticle, and membranous layer. Ultrastructurally, the exocuticle of the mill is composed of an electron-lucent matrix, pore canals, and electron-dense matrix surrounding the pore canals. The exocuticle in both cusp regions differs from that in the marginal region by the presence of a lamellar structure in the marginal region, which is composed of packed lamellae. During mineralization, needle-shaped crystals are deposited in the endocuticle. These crystals are found to consist of hydroxyapatite by electron diffraction and EDX. In conclusion, the structure of the gastric mill differs from that of the integumental cuticle, and the differences occur mainly in the exocuticle.