ABSTRACTProgressive structural changes in heated bovine semitendinosus (ST) muscle fibers were studied using phase contrast microscopy and scanning electron microscopy. ST cores were heated at two rates which simulated the oven roasting of top round roasts at 93 and 149°C to endpoint temperatures of 40, 50, 60 and 70°C. Increased heating of muscle fibers resulted in the disintegration of Z‐line structure; increases in intermyofibrillar spaces; shortening of sarcomeres; cracks and breaks in the myofibrils at the Z‐line; and fragmentation or granulation of the myofibrils. The slower rate of heating resulted in extensive granulation and fragmentation of the muscle fibers, whereas the faster rate resulted in more cracks and breaks in fibers. The effect of heating rate on the disintegration of muscle fibers suggested that the rate of heat penetration might influence the type and extent of disintegration of muscle fibers. Heating to 70°C resulted in extensive fragmentation of muscle fibers. Heat‐induced changes in the structure of muscle fibers are discussed in relation to possible influence on the tenderness of hea