This study provides a first examination of environmentally assisted fatigue crack propagation (EFCP) under atmospheric conditions and confirms addition of molybdate to chloride solution effectively inhibits EFCP in AA7075-T6 similar to or better than chromate.  Inhibition is explained by a mechanically stable crack tip film incorporating Mo species for improved passivity, and hindering production and uptake of embrittling hydrogen.  Inhibition is promoted by reduced loading frequency and stress intensity range, and potentials at or anodic to free corrosion.  These variables affect crack tip strain rate and repassivation kinetics that govern stability of the crack tip passive film.    For low R loading, a critical loading frequency exists below which the film is sufficiently stable for inhibition and inhibition increases with increasing molybdate concentration similar to chromate.  For high R loading, inhibition does not depend on molybdate concentration and effectively eliminates the environmental contribution to EFCP by producing growth rates (da/dN) typical of fatigue in high vacuum.  Molybdate inhibition depends on potential with da/dN more strongly reduced by modest anodic polarization and promoting crack arrest at ultra low frequencies.  Like the chloride-molybdate work done here, most research done has been performed under full immersion or in moist-air.  These simplified environments may not accurately simulate atmospheric conditions that include contaminants which cause deliquescence at humidities lower than expected and result in thin film electrolyte formation on the metal surface.  Experiments demonstrate enhanced Paris regime da/dN and reduced fatigue life for AA2199-T86 when exposed to a thin film electrolyte containing NaCl, but successful inhibition when molybdate is present in the surface salt.  These results suggest that a Mo-bearing coating could release molybdate that transports to a propagating fatigue crack tip for damage inhibition. This research is supported by the Alcoa Technical Center and OSD/DoD Corrosion Pilot Program on Collaborative University Research