Abstract: A retention device for inhibiting removal of carded merchandise from a display hook, adapted for retrofit installation in existing loop hooks commonly used to display such merchandise. A flat plastic body, grooved along its side edges, can be inserted between and lockingly retained by the spaced apart wire elements of the U-shaped loop hook. An integral, resiliently depressable tongue extends upward and rearward from the flat body and prevents removal of the merchandise cards unless the tongue is depressed. The tongue carries a downwardly extending abutment tab to prevent merchandise cards from being pulled underneath the tongue. The grooved side edges preferably are formed by alternating, opposed half-grooves to facilitate production by high speed injection molding processes.