The effect of mechanical treatment on the crystal structure of AgI was studied by X-ray diffraction and EXAFS techniques. It was shown that even moderate mechanical treatment of AgI results in the appearance of stacking faults in a close-packed anion sublattice. After an intense mechanical treatment the random beta-gamma polytype structure is formed. The polytypes are built of domains separated by antiphase boundaries which are the most probable sources of cationic Frenkel defects governing the high ionic conductivity and anomalous dielectric properties of mechanically treated AgI.