The structure of the ctenidium of the pearl oyster, Pinctada fucata martetnsii, was examined with corrosion casts and histological sections, and by SEM. The casts were made using the prepolymerization methyl methacrylate (MercoxCL- 2 R) containing 10 % Mercox MA. The food groove was formed by the ordinary filaments with the extended and deformed tip and the primary filaments connecting close to the center of the supra-branchial chamber side. The rows of ciliary disk run at even intervals and on a parallel with the base line of the ctenidium. The inter-laminar connecting membrane was constituted of two plate-like membranes. The inner walls of the inter-laminar connecting membrane were supported in places by the trabeculae. The lumen of the membrane connected to the whole junctional region of the primary filament vessels and to the several places of the inter-laminar connecting vessels. The vessel of the ordinary filament diverged from the longitudinal vessel passing on the base line of the lamina and branched into around the vessels of the ordinary filament. The branched vessels united into one vessel and joined to the efferent branchial vessel. The frontal surfaces of the ordinary filament and the primary filament were covered with the cilium, which were also observed on the abfrontal surfaces of both filaments. The results of histochemical staining suggested that the mucus in the mucus cell in both surfaces of the filaments probably was a sulfuric acid mucopolysaccharide.