Following the crack closure studies of Elber, experiments were performed which appear to corroborate the link between the load required to open a crack at its tip and the shift in crack growth rate curves with stress ratio. Measurements of fatigue crack growth rates and closure loads were made on compact tension specimens for positive stress ratios of R=0.08 to R=0.8 and at rates between 10 - 7 and 10 - 5 in./cycle. An R c u t exists for 2219-T851 and recrystallization annealed Ti-6A1-4V wherein da/dN versus ΔK data does not shift to faster rates with increasing stress ratios above R=0.32 and R=0.35, respectively. The physical basis for the Rent can be adequately explained in terms of crack tip closure. Fatigue S-N data also exhibited an R c u t when replotted as ΔS-N.