The temperature dependence (in the range of 479–827 K) of the fluorine-ionic conductivity of nonstoichiometric Na 0.37 Tb 0.63 F 2.26 crystals with fluorite-type structure (sp. gr. , lattice parameter a = 5.5785(1) Å) has been investigated. The crystals were grown from melt by the Bridgman method in a fluorinating atmosphere of CF 4 . The formation of cuboctahedral clusters of defects {(Na,Tb) 8 Tb 6 F 37 F 32 } is most likely in the structure of Na 0.37 Tb 0.63 F 2.26 crystals. The ionic conductivity is found to be σ dc = 2.6 × 10 –6 and 3 × 10 –3 S/cm at 500 and 827 K, respectively. A crystallophysical model of ion transport in Na 0.37 Tb 0.63 F 2.26 crystals is proposed. The concentration and carrier mobility are calculated to be, respectively, n mob = 1.45 × 10 21 cm –3 and μ mob = 1.1 × 10 –8 cm 2 /(s V) at 500 K.