The structure of zirconium silicide (ZrSi2, orthorhombic C49; a=0.369 nm, b=1.47 nm, c=0.366 nm) thin films has been investigated by high-resolution electron microscopy. The crystals are heavily faulted in the 010 plane with an average distance between faults of 1.6 nm. The fault has been found to be a π/2-rotation twin around the b axis, the habit plane lying between two silicon layers. The crystals contain an equal number of 〈a〉- and 〈c〉-oriented slabs, giving rise to a strained lattice with an average a=c lattice parameter. On the basis of an elastic calculation, the fault energy is evaluated to be of the order of 5 ergs cm -2. This low energy explains the tendency to form polytypes which are mostly disordered, although some ordered sequences are favored at short range.