Abstract Monocrystals of b.c.c. 3wt.%SiFe alloy containing preintroduced deformation twins were cyclically deformed in tension-compression under a constant stress amplitude at room temperature in air. The applied stress amplitude is 70% of the macroscopic yield stress. The surface strain markings were quantitatively examined by a combination of optical and replica electron microscopy. The bulk dislocation substructures were studied by transmission electron microscopy. Plastic deformation was localized at coherent twin boundaries; as cycling progressed, fatigue extrusions occurred at the coherent boundaries, and no changes were produced at incoherent boundaries. The localized deformation at the coherent boundaries was verified as due to to-and-fro migration of the twin interfaces themselves. The distance of the migration was found to be of the order of 0.1 μm. Finally, on the basis of these results, a way of improving the fatigue strength of materials containing preintroduced twins is proposed.