The objective of this research was to obtain and compare fracture toughness, stress corrosion cracking and constant and variable amplitude fatigue behavior of AZ91E-T6 cast magnesium alloy in both an air and 3.5% NaC1 corrosive environment. An additional objective was to determine if commonly used models that describe fatigue behavior and fatigue life are applicable to this material and test environments. Fatigue tests included constant amplitude strain-controlled low cycle fatigue with strain ratio, R, equal to 0,−1 and −2, Region II constant amplitude fatigue crack growth with load ratio, R, equal to 0.05 and 0.5 and variable amplitude fatigue tests using keyhole notched specimens. In all fatigue tests, the corrosion environment was significantly detrimental relative to the air environment. The material was also susceptible to stress corrosion cracking. Low cycle fatigue models and the Paris equation properly represented the fatigue data in both environments. Mean strains influenced fatigue life only if accompanied by significant mean stress. The Morrow and Smith, Watson and Topper mean stress models were both accurate and inaccurate. Likewise, variable amplitude fatigue life calculations using the local strain approach and based upon the formation of a 1 mm crack at the keyhole notch were both accurate and fairly inaccurate depending on the specific model used.