The contraction of vertebrate skeletal muscle is controlled by the action of Ca2+ on muscle thin filaments. At low Ca2+ concentrations (less than 10(-6)M) the regulatory proteins of the thin filament, tropomyosin and troponin, relax muscle by preventing the interaction of myosin and actin. At higher Ca2+ levels this inhibition is removed, when Ca2+ binds to troponin. Tropomyosin, a long coiled-coil alpha-helical molecule, is located in each of the two long-pitch helical grooves of actin, but troponin, a globular molecule, is attached at intervals of 38 nm (refs 1, 2). By combining evidence from X-ray diffraction studies of muscle and from electron microscopy, several authors have proposed that tropomyosin moves to block or allow attachment of myosin heads to actin. We investigate here an alternative way of combining the data, which if valid may have important consequences for our understanding of the regulation of muscle contraction.