Bovine semitendinosus muscle was heated at 50, 60 and 90°C for 45 min and was prepared for scanning electron microscopy (SEM) by freeze fracturing parallel to the long axis of the muscle fibers. Samples were dried from ethanol in a stream of nitrogen gas. Topographical features of samples, as viewed by SEM, could be identified as muscle cell components and could be compared after the different heat treatments. The effects of heat on morphology were slight at 50°C but readily apparent at 60 and 90°C. Fracture patterns were similar in raw and 50°C cooked meat, and revealed a tendency to cleave along the lengths of myofibrils. Heating to higher temperatures caused fractures to occur increasingly at fiber surfaces and at Z-lines. Endomysial collagen appeared to be unaffected at 50°C, but was congealed and non-fibrous at 60 and 90°C. The sarcolemma became granular at 60°C. The most important changes in myofibrillar structure at 60 and 90°C were an evident increase in coagulation and compactness of the A-band portion of the sarcomere and disintegration of the I-band with occasional loss of whole sarcomeres from myofibrils. Despite the obvious deterioration of structure, the overall sarcomere array remained remarkably intact. even at 90°C.