Transmission electron microscopy (TEM) examinations were made on fatigued SA533-B1 low alloy steel and Type 316L stainless steel specimens with the intention to investigate the mis-orientation changes among dislocation cells and the evolution of dislocation structures. Contrary to what might be expected for the cell structures, no clear relationship between fatigue damage and the mis-orientation changes of cell walls (or subgrain boundaries) was found in the fatigued samples of SA533-B1 steel (a bcc structure); however, significant changes of dislocation structures were observed in the fatigued samples of Type 316L stainless steel (an fcc structure). This could be accounted for by their different structures as well as complicated defect structures such as subgrain boundaries, small carbides, and dislocations inhomogeneously distributed in the SA533-B1 steel. It is interesting to note that at room temperature dislocations of fatigued SS316L specimens were observed to arrange themselves on {111} slip planes, in contrast, at 300degreesC the dislocations tend to move from their slip planes into subgrain boundaries in the surface layers rather than in the cross sectional layers. (C) 2003 Kluwer Academic Publishers.