Industrial Bombyx mori silk yarns have been degummed and mechanically tested. The principal mechanical characteristics of these yarns have been obtained and compared, before and after degumming. It has been observed that the sericin sheath surrounding the silk fibres plays a bigger part in determining behaviour than had been expected. The initial gradients of the force/strain curves were reduced by 21% when the sericin were removed and this was associated with a 20% fall in the failure forces obtained. The force/strain curves up to failure of the yarns were completely modified with the removal of the sericin and showed a multi-level fracture surface rather than a net failure. This variation was not due to the treatment involved in removing the sericin. The role of the sericin sheath has been studied using scanning electron microscopy with in situ tensile testing. The sericin have been seen to act like a matrix, as in a fibre composite with the silk fibres as the reinforcements. As in a composite, the force transfer between the sericin sheath and the inner fibre is controlled by shear forces at the interface between the two and result in the modified fracture morphology and physical properties.