Tensile and low cycle fatigue tests were conducted on Hastelloy X in vacuum at several temperatures from room temperature to 900'C. The mid-range ductility minimum generally observed in nickel-base superalloys did not show itself in the material containing yttrium and which was solution heat treated at a higher temperature. The fatigue crack growth rate was calculated from the measured intervals of fatigue striation observed by scanning electron microscopy. The specimen that was fractured at 900'C did not present the typical fatigue surface, and instead the rupture was characterized by marked reduction in area and a large cavity in the center of the fracture.