The structure of the ctenidium of the abalone, Haliotis (Nordotis) discus discus, was examined with corrosion casts and histological sections. The cast was made using the prepolymerization methyl methacrylate (MercoxCL-2 R) containing 10 % Mercox MA. The ctenidium was lateralized: the right and left ctenidia. The ctenidial plate was constructed with two walls connected to each other by numerous trabeculae. The interior spaces between the walls structured the blood vessels. The blood filling every blood vessels in the ctenidial plate flew from the afferent branchial vessel to the efferent branchial vessel. The blood flow in the ctenidium and the water flow on the ctenidium were in opposite directions. The ctenidial plate was protected by the skeletal rod in the blood vessel of the efferent side of the ctenidial plate against the hydraulic pressure generated by the lateral cilia.