Laser diffraction and photomicrography have been used to monitor sarcomere length changes in single muscle fibres of the frog, at long sarcomere lengths, during fixed end tetani. In the central 90% of all fibres, changes in sarcomere length were consistently less than 0.25 micron. Sarcomere length showed an initial rapid change, followed by a progressively slower increase, which persisted throughout a 4s tetanus. Sarcomere length in the terminal 200-400 microns segment at each end of a fibre decreased rapidly by up to 1 micron in the first second of a tetanus. This shortening was accompanied by a marked increase in disorder of the striation pattern. Maximum isometric tensions in fixed end tetani were much greater than those predicted by crossbridge theory over the entire range of sarcomere lengths studied. An analysis of the intersarcomere dynamics suggests that this extra tension may be explained by known phenomena on the basis of a progressive increase in sarcomere length dispersion along the fibre.