The fracture toughness of a nickel aluminide alloy IC-396M has been measured from room temperature to 800°C. The material was tested in the as-cast condition. Specimens were oriented for crack growth either parallel or perpendicular to the dendritic structure (C-R and R-C orientations, respectively). Both orientations had acceptable levels of toughness from room temperature to 600°C. At 650°C the toughness decreased markedly with some recovery at 800°C. Examination of the fracture surfaces revealed that the fracture mode changed from a ductile tearing mechanism at temperatures up to 600°C to interdendritic fracture at 650°C and higher temperatures. At 650°C the crack grew along the dendrite axis for the C-R specimens, but for the R-C specimens the crack grew out-of-plane in attempting to follow the dendritic structure. It is believed that the presence of the sharp crack in the fracture toughness specimens allowed oxygen to penetrate to the crack tip, where straining exposed metal surfaces that were not protected by an oxide layer. The result was embrittlement of the interdendritic regions and low energy fracture, analogous to intergranular fracture in wrought materials.