Extended crystal lattice defects in tetragonal SrLaGaO4 single crystals were investigated by X-ray diffraction transmission and back reflection projection topography. The crystals were grown by the Czochralski technique using a [001]-oriented seed in nitrogen atmosphere with various oxygen contents. Crystals grown in atmosphere with higher oxygen content (7.5 x 10(3) ppm) exhibited many defects in the region surrounding the core-linear black diffraction contrasts and volume-defect contrast of irregular shapes. The central part of the sample (core) was practically free from defects, only short linear diffraction contrasts were detected. Crystals grown in an atmosphere with lower oxygen content (1.7 x 10(3) ppm) contained only linear defects in the core region and other contrasts in the outer region, probably associated with dislocations. (c) 2006 Elsevier B.V. All rights reserved.