Sustained load creep and fatigue crack growth tests have been conducted over a temperature range of 870 to 1038°C for single-crystal (SC) Renè N4 nickel-base superalloy.Tests were conducted in both laboratory air and an inert environment of a vacuum of 10-5 torr under similar conditions to understand the role of environment in elevated temperature crack growth behavior of this material. Mini-compact tension type specimens were fabricated with orientation  parallel to the loading axis and  direction parallel to the crack growth direction. The creep crack growth rate in laboratory air at 870°C was found to be nearly an order of magnitude higher as compared to tests conducted in a vacuum of 10-5 torr. However, in fatigue at a stress ratio of 0.1 and a frequency of 1.0 Hz, the crack growth rate was found to be somewhat higher in vacuum as compared to growth rates obtained from tests in laboratory air. A number of constant K tests were conducted with different frequencies of 0.1 and 0.01 Hz and several hold-times to investigate the time-dependent effects. Fractographic studies of the fatigue crack growth specimens tested in laboratory air and vacuum did not show any distinct differences in failure mechanism. However, interdendritic failure was observed for creep specimens tested in laboratory air, while the fracture surface obtained from vacuum tests showed crystallographic features.