Cross-striated muscle fibers may contract reversibly to less than 30 percent of their rest length and it is not easy to reconcile this fact with the sliding filament model of muscular contraction. The mechanism of supercontraction has been studied in fibrils obtained from the giant muscle fibers of the barnacle Balanus nubilus. They were examined by phase-contrast light microscopy and electron microscopy. Contraction beyond the 50-percent stage was found to be achieved largely by the passage of thick filaments through the Z-disks, which are perforated. The overlap of thick filaments from adjacent sarcomeres causes the appearance of the contraction bands about the Z-disks. Subsequent contraction is associated with a folding and loose coiling, but not a shortening, of the thick filaments.