AbstractThe dislocation structures formed in the plastic wake of a long fatigue crack have been characterized by TEM for polycrystalline copper tested in air and in vacuum. In order of increasing distanceDfrom the fracture surface, typical structures are cells, walls, PSBs and veins, and finally loop patches and dislocation tangles. The outer boundaries of these different regions depend on the stress intensity factor range ΔK.By comparison with the dislocation structures developed in LCF specimens fatigued at a constant strain amplitude, the structures in the plastic wake appear much distorted and more than one set of slip systems are often found to be activated. This feature is the consequence of the complex stress history occuring in a unit volume of material located at a distanceDfrom the fracture surface. Due to this fact, the plastic strain gradient deduced from the size of dislocation cells can only be approximated. The dislocation structures formed near the fatigue crack tip in vacuum appear to be somewhat different from those formed in air; the cell boundaries are much denser, sharper and straighter, the misorientations between them are more marked and, at a given distanceDand for a given applied ΔK, the cell size is also smaller than that formed in air. These effects are the consequence of the larger number of cycles that a unit volume of material located at a given distanceDcan sustain during fatigue cracking in vacuu