This report presents the results of an investigation continuing an examination of the fatigue properties of aluminum in vacuum. In the current phase, experimental work was directed at cyclic frequency and tensile fatigue effects and at the defect substructure generated in vacuum by fatigue stressing. The mean fatigue life was observed to depend on the cyclic rate of straining at all pressure levels. However, in the region of the critical pressure zone for fatigue life enhancement, changes in the cyclic frequency were noted to have a major effect on crack propagation, an increase in cyclic straining rate acting to shift the transition level to higher pressures. Measurements of the fatigue life under conditions limited to alternating tensile stresses alone showed that fatigue life enhancement or crack growth retardation in vacuum was not dependent on compressive stress distributions or fatigue crack closure. The experimental work inferred that crack retardation should not be mainly attributed to the reweldment of fracture surfaces in vacuum, but may be produced by dislocation glide and escape at oxide-free surfaces to reduce internal stresses. Finally, a well defined correlation was observed between the fatigue properties of aluminum and dislocation substructural distributions revealed by thin film electron microscopy. Under conditions of repeated loading, subgranular dislocation arrays were formed with an average size dependent on the strain amplitude and the prior dislocation distribution.