Angling for Muskellunge Esox masquinongy has become widespread and increasingly popular. Management strategies for Muskellunge include high minimum harvest lengths (>1016mm), closed seasons, and catch-and-release regulations. Due to these strategies and angler behaviors, up to 97% of Muskellunge caught are released, and anglers assume these fish survive to be caught again in the future. Previous research on catch-and-release mortality for Muskellunge has suggested relatively low mortality rates (0%-5%). However, these studies were all conducted within the range of water temperatures that are thermally optimal for Muskellunge and generally at water temperatures <25°C. Muskellunge populations in some latitudes routinely experience temperatures >25°C during the summer months. Fisheries managers and anglers have expressed concerns regarding warm water angling mortality, representing a need to evaluate mortality rates at various water temperatures and multiple latitudes. Our objective was to quantify warm-water (>25°C) catch-and-release mortality rates in Muskellunge (>760mm) and identify factors influencing mortality using experimental ponds. Adult Muskellunge (n=102) were stocked into eight earthen or plastic lined flow-through ponds (0.06-0.71 ha) at densities of <16 fish/ha. Muskellunge (n=50) were angled utilizing specialized Muskellunge fishing gear at water temperatures of 19.6–32.6°C, with 32 fish being caught at temperatures >25°C. Fish were closely monitored for 2 weeks after being angled to assess mortality, and fish that remained uncaught during the experiment were used as controls (n=53). Mortality was greater for angled (30.0%) compared to control fish (11.3%). Differences in catch-and-release mortality were compared across a range of temperature regimes using firth's logistic regression. Five-day cumulative temperature and net time were positively related to the probability of mortality, but size and sex were unrelated to mortality. Elevated catch-and-release mortality rates at warm water temperatures warrant investigations into the population level effects at varying levels of exploitation.