From the study of 6H and 4H single crystals, grown under various conditions on Si- and C- terminated seed surfaces, two types of defects with different slit length are discussed. Short slits with a length in the submillimetre range are spread between micropipes in an advanced growth stage. Their generation is correlated to micropipe agglomeration. The long slits up to some millimetres, which are associated with dislocation bundles in the basal plane, are correlated to polytype domain walls. In this region the slit formation seems to be favourable similar to the Frank mechanism of micropipe generation (Acta Crystallogr. 4 (1951) 479). (C) 2002 Elsevier Science B.V. All rights reserved.