Comprehensive studies have been completed of the fatigue crack growth behavior of aluminum-magnesium alloy 5083 in vacuum, air, and NaCl solution, for both unsensitized and sensitized conditions. At low load ratio, R, sensitization has no effect on the fatigue thresholds in vacuum, air or NaCl, and only small effect on crack growth rates. At high R, fatigue in air and vacuum does not depend on sensitization at all. However, in NaCl solution, the high R fatigue behavior depends very strongly on sensitization. To reconcile all observations, we consider a model of two superimposed environment-enhanced cracking effects: a purely cyclic mode acting in both air and NaCl solution, and a monotonic stress-corrosion cracking mode acting at high stress intensity only in NaCl. The model details are described, and comparison with experiment as a function of load ratio, crack growth rate, and degree of sensitization is discussed.