Threading dislocations in wurtzite ZnO films grown on the (11 (2) over bar0) a plane of sapphire were studied by transmission electron microscopy. A majority of the threading dislocations were found to be of screw or mixed character. Dislocation half loops, elongated along the c axis, were observed. It is likely that they are formed when two screw dislocations of opposite sign attract each other during growth and combine.