The phenomenon of contraction of a striated muscle down to below 50 per cent rest length has been examined for the scutal depressor of the barnacle Balanus nubilus by a combination of phase contrast and electron microscopy. It was found that neurally evoked contraction down to 60 per cent rest length results from the shortening of the I band. At the same time the Z disc changes in structure by an active process which results in spaces opening up within it. Thick filaments can now pass through these spaces from adjacent sarcomeres, interdigitating across the discs. Interdigitation permits repetitive contraction in the living muscle to below 30 per cent rest length. In non-neurally evoked contractions most thick filaments do not find spaces in the Z disc and bend back, giving rise to contraction band artifacts. Expansion of the Z disc can be produced in glycerinated material by the addition of solutions containing a high concentration of ATP.