The ultrastructure and localization of supercontracted sarcomeres during the spreading (Zenker's) necrosis of twitch and tonic frog fibres and the fast- and slow-twitch rat fibres are described. The formation of supercontracted sarcomeres proceeds mainly by the sliding mechanism. Contracted bands are characterized by homogenization of myosin and actin filaments and by the disappearance of Z-band structures. In the calcium-free medium some myosin filaments pass through the whole depth of the contracted band or are seen bending and deviating backwards upon approaching the contracted band. In calcium-free solution supercontracted sarcomeres often form narrow strips (2-5 sarcomere long), crossing the fibres. The transverse elignment of sarcomeres is typical of supercontracted portion of fibres.