We studied by high-resolution immunofluorescence (HRI) and by confocal laser scanning optical microscopy (CLSOM) the costameric organization of dystrophin and vinculin at the surface membrane of muscle fibers from 4 young boys with Becker muscular dystrophy (BMD). By HRI, the surface membrane of normal fibers showed regular parallel bands encircling the fiber at the level of I and M band. In BMD fibers, the dystrophin bands were stretched apart, interrupted, or did not show the intermediate band encircling the M band. By CLSOM, computer reconstruction of muscle surface membrane showed disorganization of the costameric dystrophin lattice at the membrane level in BMD muscle, in contrast with the preservation of the costameric lattice organization of vinculin.