HUMAN X-linked Duchenne and Becker muscular dystrophies are due to defects in dystrophin, the product of an exceptionally large gene^1,2. Although dystrophin has been characterized as a spectrin-like³ submembranous⁴ cytoskeletal protein, there is no experimental evidence for its function in the structural maintenance of muscle⁵. Current hypotheses attribute necrosis of dystrophin-less fibres in situ to mechanical weakening of the outer membrane⁶, to an excessive influx of Ca²⁺ions^7,8, or to a combination of these two mechanisms, possibly mediated by stretch-sensitive ion channels⁹. Using hypo-osmotic shock to determine stress resistance¹⁰ and a mouse model (mdx)^11,12 for the human disease, we show that functional dystrophin contributes to the stability of both cultured myotubes and isolated mature muscle fibres.