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Structural characteristics of aged (10 days) bovine longissimus and semitendinosus muscles as affected by thermal treatment were evaluated with scanning electron microscopy (SEM) and transmission electron microscopy PEM). Steaks were oven roasted at 177°C to internal temperatures of either 63, 68, or 73°C. At 63°C both muscle samples exhibited coagulation and denaturation of the perimy-sium and sarcolemma and progressively increased to near complete coagulation and denaturation at 73°C. Well-defined myofibrillar surface features were present in semitendinosus samples at 63°C and increased sarcomere denaturation and shrinkage were observed at the higher temperatures. Whereas, the overall topographical features of longissimus myofibrils were not well-defined and exhibited a granular appearance even at 63°C. TEM of samples heated to 63°C revealed less distinct myofibrils in longissimus than in semitendinosus samples. Disintegration of filaments in the I-band and shrinkage of filaments in the A-band occurred at 63°C. Increased disintegration of actin filaments in the I-band at the junction of the Z-disc and shrinkage and disruption of the A-band material occurred as temperatures were increased. Z-disc material remained intact with some evidence of disruption at the higher temperatures. Although progressive disruption of muscle fiber ultrastructure occurred as steaks were heated to higher internal temperatures, these changes did not result in increased muscle tenderness. Instead the samples became progressively less tender as internal temperatures were increased likely due to the shrinkage and hardening of filamentous material in the A-bands. Increased disintegration of filaments in the I-band as temperature was increased apparently did not contribute to an improvement in tenderness.