SUMMARY:Susceptibility of chicken pectoralis major myofibrils to mechanical fragmentation was investigated, after various periods of aging, as an index of tenderness. Treatments were used which accelerated, retarded or prevented postmortem glycolysis. Fragmentation, with breaks always beside the Z line, was measured by microscopic examination of homogenized muscle. Fragmentation of pre‐rigor muscle produced small, contracted and poorly defined particles. As rigor mortis developed, fragments became longer, more rigid and clearly defined. With additional aging after full rigor. homoaenization produced progressively smaller myofibrillar fragments consisting of 1 to 6 sarcomeres. Muscle was feast tender (as measured by shear force) when it was in full rigor and tenderized with subsequent aging. Retardation of the onset of rigor mortis extended the time required for tenderization. Although the fragmentation pattern generally corresponded to changes in tenderness in glycolysing muscle, fragmentation was not found to be an accurate index of tenderness. Sarcomeres did not lengthen during the aging period after rigor mortis develope