Previous work has shown that in frog striated muscle fibers there is a mechanical coupling between the sarcolemma and the column of interdigitating arrays of myofilaments. An ultrastructural study will be made of myofibers, at points where special stress or strain of this coupling can be expected, in an attempt to find an anatomical basis for it; points such as highly stretched fiber tips and the attachments of the festoons of the sarcolemma in shortened fibers. The mechanical strength of coupling and its capability for transferring active tension are being tested in single myofiber preparations where differential stretch or shortening of fiber segments results in segmental inactivation of the myofilament column.