SUMMARY—Pre‐rigor and post‐rigor semitendinosus muscle was examined under the light microscope and in the electron microscope before and after cooking. During cooking, pre‐rigor samples were unrestrained or restrained on stainless‐steel frames at normal length and at 130% normal length. Samples which passed at 0°C into rigor unrestrained, or while restrained to the same degrees of stretch as above, were cooked post‐rigor without mechanical restraint after various periods of storage. The treatments produced differences in tenderness of the muscle pieces which were related to differences seen in the ultrastructure in the micrographs. It was concluded that, provided the effects of gross connective tissue are small, tenderness in beef muscle is produced by disruption of the actin filaments and by a breaking down of the linkages between the actin and myosin filaments in the sarcomeres.