Monomethylurea single crystals grown by mechanical dimension limitation and top seed solution growth methods have been characterized by X-ray topography and high resolution X-ray diffraction techniques. The samples showed a relatively low defect content with typical values of the full width at half maximum of the X-ray diffraction peaks of 15 arcsec. X-ray topographs using the Cu K alpha (0 2 2) transmission setting evidenced that the seed-crystal interface is a very critical region giving rise to the formation of inclusions and grown-in dislocations. The absence of dislocations revealed by X-ray topography in top seed grown crystals confirmed the possibility of obtaining large volumes of crystal nearly dislocation free. The preparation of organic crystals for X-ray topographic characterization requires a careful preparation of surfaces. In monomethylurea single crystals this process is very critical leading to the formation of surface defects which partly mask the topographic contrast from grown-in defects.