Abstract: A security nut and hook assembly compatible with pipe-type car top carriers in order to prevent inadvertent and unauthorized removal of a payload from such a carrier includes a hook-forming element arranged for engaging a rail of the car top carrier. A clamp engages with the hook-forming member for securing a payload to the rail of the carrier, with a retainer attachable to the hook-forming member and the clamp for biasing the clamp against the rail of the carrier while simultaneously retaining the entire assembly on the carrier rail.