Zircaloy-2 sheet materials of thickness 2 mm have been tested in an air, ethanol and ethanol-containing-iodine (at 0.1 vol%) environment at different loading frequencies between 0.1 and 0.3 Hz in a tension-tension, constant displacement type fatigue machine at maximum load ( P max) = 0.1 and 0.15 breaking load (BL) and initial stress ratio, R(= minimum stress maximum stress = σ min σ max ) = 0.1 . Susceptibility, as was determined by crack velocity (CV) and number of cycles to failure criteria, has been indexed as: ethanol + iodine > ethanol > air. Increased susceptibility was accompanied by an increasingly brittle fracture mode. Crack velocity increased with increasing P max and stress cycle frequency. Irrespective of the test variables used, flattening out of the P max vs number of cycles to failure curve at p TH was postulated as the threshold condition, concomitant with the observed transition in the fracture mode brittle-ductile. Tests in air had revealed a continuously sloping downward crack progression curve with an increasing number of cycles, while P max had decreased continuously. The fracture mode was microvoid coalescence (MVC).