A solid rubber cylinder with metal end plates fractured in the rubber when a torsional deformation was applied 'which corresponded to a shear angle of 56° on the cylinder surface. The height of the cylinder was maintained accurately constant during the deformation. A comparatively smooth fracture surface was created in a direction perpendicular to the principal tensile stress and a rough surface was created perpendicular to the principal compressive stress. The markings on the smooth surface have some feature similar to a cleavage-type failure in a crystalline material.