Crystal lattice defects in GdCa4O(BO3)(3) (GdCOB) single crystals were investigated for the first time by transmission X-ray topography. Straight and curved dislocations were observed in samples cut perpendicular to the crystal growth axis b and parallel to it. A large, spherical inclusion was found in the sample cut perpendicular to the [0 10] growth axis. Bundles of dislocations were detected in the sample cut parallel to the crystal axis b. A set of dislocations appearing in the vicinity of the spherical stress center was observed too. Etch pits, attributed to dislocations and observed on (0 10) and (10 0) planes, reflect the symmetry of the etched crystal planes. Comparison of the etch pits optical pattern of the sample cut perpendicular to the crystal growth axis b with its transmission topograph enable to state the nature of dislocation arrays that are detected by chemical etching. (C) 2003 Elsevier B.V. All rights reserved.