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Cyclic deformation behaviour of a [(4) over bar 15 20]-[18 (2) over bar 7] copper bicrystal with a tilting Sigma = 19b grain boundary (GB) was investigated in the axial plastic strain range 1.5 x 10(-4) -2.13 x 10(-3) at room temperature in air. The primary slip planes (111) within the G(1)[(4) over bar 15 20] and G(2)[18 (2) over bar7] grains in the bicrystal were designed to be coplanar in order to reveal the interaction of slip bands with the GB. The results show that the cyclic stress-strain curve of the bicrystal displays a plateau region with axial saturation stresses of 61.6-63.5 MPa over the applied strain range. This result is similar to that for a single-slip-oriented copper single crystal, indicating that the GB has little effect on the saturation stress. After cyclic deformation, the surface morphology of the bicrystal exhibits the following features. Firstly, only the primary slip system was activated in both grains. Secondly, the primary slip bands on the four surfaces of the two grains show a good one-to-one relationship across the GB, indicating that surface slip bands can transfer through it. Thirdly, secondary slip systems were not activated, even in the vicinity of the GB. Dislocation patterns of the bicrystal were observed by the electron channelling contrast technique. The two-phase structure of persistent slip bands (PSBs) and matrix (or veins) formed in both grains. The ladder-like PSBs were observed to transfer through the GB continuously on one surface of the bicrystal but piled-up at the GB on the other surface, showing a discontinuous dislocation distribution next to the GB. Several kinds of interaction between dislocations and the GB were observed on the common slip plane. The cyclic stress-strain response and the interactions between dislocations, PSBs and the GB are discussed.