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Understanding the temperature tolerances of organisms is critical because the thermal regimes of freshwater ecosystems are changing globally. Native freshwater mussels are sensitive to increasing water temperatures because of their physiology and unique life history. Detailed knowledge on lethal temperatures for mussels has been limited to less than 5% of the species known to occur in North America, and little is known about the thermal tolerances of mussel species from rivers within the south-western USA. To determine the effects of elevated water temperature on mussels, the upper thermal tolerances of larvae (glochidia) for the following species across four basins in Texas (Neches, Guadalupe, San Antonio, and Colorado) were tested: Amblema plicata, Cyclonaias necki, Fusconaia mitchelli, Lampsilis bracteata, Lampsilis hydiana, Lampsilis satura, Lampsilis teres, and Obovaria arkansasensis. Glochidia were acclimated to 27 degrees C across a range of experimental temperatures (30-39 degrees C) in 24-h standard acute laboratory tests. The median lethal temperature (LT50) among glochidia averaged 32.4 degrees C and ranged from 26.9 to 36.4 degrees C. Thermal tolerances differed significantly among and within species, and by season. Comparing these results with current water temperatures in central and east Texas indicated that populations of the focal species studied are at risk from rising environmental temperatures and, as a consequence, their long-term viability will be challenging in future years.