Bulk single crystals of 4H-SiC have been deformed in compression in the temperature range 550–1300°C, whereas a GaN thin film grown on a (0001) sapphire substrate was deformed by Vickers indentation in the temperature range 25–800°C. The TEM observations of the deformed crystals indicate that deformation-induced dislocations in 4H-SiC all lie on the (0001) basal plane but depending on the deformation temperature, are one of two types. The dislocations induced by deformation at temperatures above ∼1 100°C are complete, with a Burgers vector, b, of but are all dissociated into two partials bounding a ribbon of stacking fault. On the other 3 hand, the dislocations induced by deformation in the temperature range 550<T<∼ 1100°C were predominantly single leading partials each dragging a stacking fault behind them. From the width of dissociated dislocations in the high-temperature deformed crystals, the stacking fault energy of 4H-SiC has been estimated to be 14.7±2.5 mJ/m2. Vickers indentations of the [0001]-oriented GaN film produced a dense array of dislocations along the three 〈1120〉 directions at all temperatures. The dislocations were slightly curved with their curvature increasing as the deformation temperature increased. Most of these dislocations were found to have a screw nature with their b parallel to 〈1120〉. Also, within the resolution of the weak-beam method, they were not found to be dissociated. Tilting experiment show that these dislocations lie on the {1100} prism plane rather than the easier (0001) glide plane.