The breakdown of sarcomeres of frog's twitch skeletal muscles during Zenker's (spreading) degeneration has been studied. The speed of propagation of the destruction process was accelerated by increasing CaCl2 concentration in Ringer's solution up to 8mM. An hour after local injury, the fibres were fixed just before separation of the next retraction clot or at the stage of granular destruction (Fig. 1). The dominating features of the ultrastructure of a fibre at the necrotic boundary are the coagulation of small bundles of supercontracted myofibrils and breakdown of uncontracted sarcomeres into separate A- and I-bands and then into small bundles of A- and I-protofibrils (Fig. 2,3). The same breakdown of sarcomers is observed in several small regions at a distance of about 100 micron from the necrotic boundary (Fig. 5). Besides this, fusion of a few myofibrils followed by the disappearence of M- and Z-bands occurs in the same region of the fibre (Fig. 4, 6). The diameter of the majority of myofibrils decreases towards the necrotic boundary due to longitudinal splitting and loss of peripheral protofibrils, presumably, as a result of lysis (Fig. 7).