Critical thermal maximum (CTM) and resistance time to high temperature were determined for juvenile Arctic grayling Thymallus arcticus from the fluvial population of the Big Hole River, Montana. Grayling were tested after acclimation to 8.4, 16.0, and 20.0°C. Thermal tolerances increased with acclimation temperatures; mean CTM was 26.4°C for the 8.4°C acclimation group, 28.5°C for the 16.0°C group, and 29.3°C for the 20.0°C group; median resistance time at given test temperature also increased with acclimation. The upper incipient lethal temperature (UILT) was 23.0°C for fish acclimated to 8.4°C and 16.0°C, and was 25.0°C for those acclimated to 20.0°C, temperatures that were similar to the median tolerance limits of Arctic grayling in Alaska. Comparisons of mean CTM and UILT for juvenile Arctic grayling with levels and durations of maximum river temperatures recorded during summers 1992–1994 indicated that resident fish may occasionally be subjected to potentially lethal temperatures in the warmest reaches of the Big Hole River.