Both opening-mode and mixed-mode fracture toughness tests were carried out at 1200 and 1300 degrees C on a sinter/HIP grade of silicon nitride. Data for pure opening loading (K-Ic) agree well with other experiments on the same material, which showed that the toughness was lower at 1000 degrees C than at room temperature, but increased as temperature increased above 1000 degrees C. The ratio of K-IIc/K-Ic was sufficiently insensitive to temperature that it can be considered to be constant. Results are discussed in the context of mechanisms that have been proposed to explain fracture toughness in silicon nitride.