This paper presents the structural details of nylon 66 transcrystallinity induced by aramid (kevlar 29, 49 and 149) and carbon (pitch based) fibers, as determined by high spatial resolution X-ray diffraction. Using stepwise scanning, the orientation of the lamellae in the transcrystalline layer was measured as a function of distance from the fiber. The main observation is that this orientation is distinct for each system and almost independent of distance from the fiber. Of particular interest is the bi-layered transcrystallinity formed on a surface treated kevlar 49 fiber, in which the lamellar a* axis is nearly perpendicular and at an angle of similar to 12degrees to the fiber in the outer and inner layer, respectively. The crystallographic analysis generates grids of oriented lamellae with respect to the fiber axis. (C) 2004 Elsevier Ltd. All rights reserved.