The torsion of box beams of rectangular section, the edges of which are strengthened by flanges, and of which one side is lacking, is analysed by the energy method. The torsional stresses are generally taken up by the bending of the two parallel walls, the rigidity of which is augmented by the third wall. The result was checked experimentally on duralumin and plywood boxes. The torsion recorded was 10 to 30 per cent, less than that given by the calculation, owing to self-stiffening.