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The cyclic stress-strain response and corresponding fatigue dislocation structures of a <[(3)over bar 45]>/<[(1)over bar 17]> copper bicrystal were investigated over a shear strain amplitude range from 0.28 x 10(-3) to 6.45 x 10(-3). Similar to copper polycrystals, this bicrystal was found to skew very high saturation stresses and no obvious plateau but a bulge in the cyclic stress-strain (CSS) curve. The dislocation structures in the fatigued bicrystal were also found to be similar to usually observed in cyclically deformed copper polycrystals and multiple slip copper single crystals. However, the dislocation structures in the two component crystals were different from each other. At low strain amplitudes, the prevailing dislocation structure in the <[(3)over bar 45]> component crystal is loop patches, while the typical dislocation structure in the <[(1)over bar 17]> component crystal consists of labyrinth-like loop patches and less orderly persistent slip band (PSB) ladder structures. With increasing strain amplitude above 0.89 x 10(-3), cell structures and labyrinth structures were found in the <[(3)over bar 45]> component and in the <[(1)over bar 17]> component, respectively. The labyrinth structure was found to contain (001) and (210) walls perpendicular to each other. It was also found that the grain boundary (GB) acted as an obstacle to the motion of dislocations. The dislocation-free zones, which were reported beside the GBs in fatigued copper polycrystals, were not observed in the present fatigued bicrystal. The formation of labyrinth structure was believed to be caused by the interaction of dislocations with two orthogonally oriented Burgers vectors. The occurrence of multiple slip in the two components due to the constraint of GB is interpreted to cause the observed fatigue deformation behavior and dislocation structures. (C) 1997 Acta Metallurgica Inc.