The deformation energetics of static polyethylene chain defects were determined by means of molecular mechanics and energy minimization. The eight defects considered in our study were dispiration and dislocation (both interstitialand vacancy-like), disclination, chain twist boundary (90°a nd 180°), and partial dislocation boundary. For these defects, energy minimized structures were calculated under action of tensile forces. It was found that one group of defects had moduli and deformation energetics identical with those of an all-trans chain, whereas the other group of defects had substantially lower moduli and a different deformation behavior. From the analysis of the distribution of deformation energy over the several degrees of freedom and by comparison to an all-trans chain, it could be concluded that defects inside a crystalline matrix are not weak links in polyethylene fibers. Weak links were concluded to be chains having higher moduli than the surrounding matrix (for example, taut tie molecules and especially entanglements in amorphous domains).