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The torsion test to failure is described for a monocoque box of 249-T aluminum alloy, The box had a rectangular socticn and was reinforced by Z-stringers, antirolls, bulkheads, and corner posts. The twist and strains in the stringers, sheet, corner posts, and bulkheads were measured for loads practically up to failure. Failure occurred by tearing of the cover sheet subsequent to failure of rivsts joining antfrolls to corner posts. The buckling loads of the cover sheets ' and of the shear webs agreed width the computed values, The measured twists---agreed within 6X 1We radian per inch with those computed from a theory which treats the box as an assembly of four beams with wide webs, joined at the extreme fibers; the ef-fect of buckling is taken into account in the theory by trcat-ing the web as a diagonal tension field following Wagner czd Langhaar. The measured stresses in the shoet were scmewhnt smaller than those indicated by the theory, The theory indicates the prceence of excessive shearing stress before failure in the rivets joining the antirolls to the corner posts..&gt