Considerable attention has been given to the potential impacts of global climate change on biodiversity. In the present study, we combine understudied themes by examining the ability of a freshwater fish (polymorphic for heat-sensitivity) to respond to short-term thermal stress mimicking an extreme temperature event. We simultaneously measured the effect of thermal stress on the body condition of heat-sensitive and heat-tolerant forms to evaluate an existing hypothesis regarding the underlying mechanism by which temperature affects the maintenance of genetic variation in this species. Surprisingly, the heat-sensitive allelic variant increased in body condition equally as much as a heat-tolerant variant under acute heat stress. More importantly, the heat-sensitive variant exhibited a significant response to thermal stress, with an upward shift of greater than 2 °C in critical thermal maximum. Our findings suggest a complexity to the relationship between thermal stress and male body condition that may depend on an interaction with other factors such as resource level. Although the evolutionary fate of species with respect to climate change is typically evaluated in terms long-term adaptive response, short-term selection events could drastically reduce fitness and reduce evolutionary potential. Our results suggest that heat-sensitive species may have considerably greater resilience to the short-term, extreme perturbations to the environment that are expected under climate change. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 504–510.