Anion voidage and anion void lattice formation in electron irradiated (100 keV–1 MeV) fluorite (CaF2) has been investigated byin situtransmission electron microscopy. At low fluences (≲1 dpa) the damage consists of randomly distributed anion voids and large bubble-like defects associated with fluorine gas. At higher fluences (≳ 1 dpa) the anion voids form a three-dimensional ordered superlattice with asimple cubicstructure, whose axes are commensurate with those of the matrix. The superlattice has a unit cell parameterA0in the range 15–30 nm and a ratioA0/R(superlattice parameter/anion void radius) in the range 3–6. The fully ordered superlattice “crystallizes” from randomly distributed anion voids by an ordering process in which locally ordered areas grow together after a fashion similar to that of epitaxy, leaving distorted superlattice regions between individual, fully ordered blocks.