In the eastern Pyrenees mylonite belts are related to a late phase of Hercynian folding leading to the development of an open antiform-tight synform large-scale structure. These belts consist of steep dipping and anastomosing bands ranging in width from a few centimetres to some tens of metres, broadly parallel to the folds and preferentially developed in crystalline rocks on the cusped synformal hinges. The mylonite bands are zones of ductile high deformation related to displacements essentially parallel to the banding. The attitudes of the stretching lineations in the mylonites indicates that dip-slip dominates in the E-W trending mylonitic bands while strike-slip becomes common in the NW-SE trending zones. The mylonitic bands correspond to shear zones which have an opposite sense of displacement on each limb of any one fold. The geometry of the individual banded structures is dependant on both the degree of the pre-existing anisotropy and its orientation. In isotropic crystalline rocks the mylonite bands correspond to simple shear zones, while in well-foliated schists these form transposition bands related to stretched and thinned limbs of asymmetric folds. A gradation between the two types exists. Increasing crystallinity with depth causes a change in structural style of late Hercynian deformation from fold related banded structures to ductile shear zones. © 1980.