Heated sphalerite and natural disordered wurtzite from Pribram, Czechoslovakia have been studied by high resolution electron microscopy. The sphalerite heated at 950°, 1000° and 1050°C commonly shows 2H, 4H and twinned 3C structures with a few 12R and 15R polytypes. The 4H polytype may be stable only in a limited temperature range. ZnS specimens from Pribram consist only of disordered 2H and 3C sequences, no 4H sequences were found. A stacking sequence due to so-called isolated or coupled partial dislocations was not found in either of heated sphalerite or natural wurtzite. The original sphalerite and wurtzite were deformed by local thermal strains at an early stage of the phase transition, and then recrystallized at the final stage, resulting in a considerably ordered structure.