The elastic-plastic J_1c fracture toughness response of precipitation heat-treated Inconel X-750 was evaluated by the multispecimen resistance-curve (R-curve) technique at room temperature and elevated temperatures. The fracture toughness of this nickel-base superalloy was found to increase slightly under intermediate temperature conditions (427°C), but it decreased dramatically at the highest test temperature (649°C). Metallographic and electron fractographic examination of fracture surfaces revealed that the slight increase in J_1c at intermediate temperatures was associated with an intergranular-to-transgranular fracture mechanism transition. At room temperature, crack extension occurred primarily by an intergranular dimple rupture mechanism attributed to microvoid coalescence along a grain-boundary denuded region. In the 427 to 538°C regime, the fracture surface was dominated by a transgranular morphology consisting of poorly defined dimples coupled with a rather flat, faceted appearance. At 649°C, intense heterogeneous slip initiated localized separation, which resulted in a very faceted fracture surface morphology and a severe degradation in fracture toughness.