A fitting for trimming the horizontal stabilizer of an aircraft, made of composite material, the fitting including side walls of a torsion box as well as joining elements which join the fitting to the frames of the tail fuselage of the aircraft, the side walls being joined together by a central element which includes a first end part joined to the first side wall of the fitting, a second end part joined to the second side wall of the fitting and a central part which joins together the end parts, the fitting includes end elements which are joined to the side walls on their outer face, the fitting having, owing to its greater rigidity in response to side load and vertical load stresses which tend to close the side walls, its greater integration and the simplicity of the load path, an optimum structural behavior in response to the aircraft stresses.