Macroscopic crack growth rates, da/dN, have been obtained as a function of the stress-intensity factor range, ΔK, for Type 2219-T851 aluminum in ambient air at load ratios, R, of 0.1 and −1. Microscopic modes of crack extension were also determined by transmission electron fractography. Fatigue striation spacings, S, compared well with da/dN when static fracture modes—primarily dimpled rupture—did not occur. These static fracture modes were more prevalent during tension-tension loading at R = 0.1 than during tension-compression loading at R = −1, and they contributed to higher growth rates at the positive load ratio. The relationship between striation spacing and ΔK/E (where E is the elastic modulus) in Type 2219-T851 aluminum was found to be similar to those for other engineering alloys. This relationship was observed to be much less sensitive to changes in the load ratio than the relationship between da/dN and ΔK.