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Freshwater mussels are ecologically and culturally important and globally have the highest extinction and imperilment rates of any animal. The Western Ridged Mussel (Gonidea angulata) and the Western Pearlshell (Margaritifera falcata) are experiencing die-offs across their native ranges, and increasing water temperature is a possible contributing factor. To host their larval life stage, G. angulata uses the gills of sculpins (Cottus spp.) and M. falcata uses the gills of salmonids (Oncorhynchus spp.). The purpose of this laboratory study was to determine the upper thermal tolerances of G. angulata and M. falcata and two sculpin species, Margined Sculpin (Cottus marginatus) and Paiute Sculpin (Cottus beldingi), by using sublethal acute and chronic survival tests. The acute test consisted of increasing the water temperature until critical thermal maximum (CTmax) temperature behaviors were observed. The mean CTmax temperature was 34.1°C for G. angulata and 33.2°C for M. falcata, and the mean for the two Cottus spp. was 28.9°C. Based on the acute test results, G. angulata and M. falcata have a higher CTmax temperature than their host fish. The chronic test gradually increased the water temperature surrounding the test organism to 2°C below the CTmax temperature for each species. This temperature was maintained for 7 d, during which survival was recorded. All mussels perished, whereas 58.3% of the sculpins survived the chronic test. Survival curves from the chronic test showed that G. angulata survived longer and are more tolerant to prolonged elevated temperatures than M. falcata. These thermal tolerance tests demonstrate that a thermal separation between mussel and host fish is possible, thus limiting mussel reproduction as climate change leads to warming surface waters. In addition, tolerance to prolonged elevated temperature varies between these western mussel species.