High-temperature fatigue of Ni-based single crystal superalloys is studied at 1000 °C in a wide range of loading conditions (−1 ? R ? 0.8) and number of cycles (103 − 109). Under fully reversed conditions, a competition between crack initiations from the surface— assisted by oxidation—and from internal metallurgical defects—mostly large casting pores—is observed. Increasing the testing frequency shifts the competition to a higher number of cycles. Conversely, decreasing the casting pore size or coating the specimen promotes surface initiations. When a positive mean stress is added (R ? 0), a creep deformation/damage mechanism mainly controls fatigue life, despite fracture surfaces presenting a variety of initiation types. Fatigue life can be predicted by a simple creep law if the contribution of the alternating stress is considered. A linear damage summation method that considers pure fatigue and pure creep damage is used to predict the fatigue lives, and Haigh diagrams for different alloys are presented.