Global climate change caused by human activity over the course of the last hundred years has altered our natural environment. One of the largest anthropogenic changes is increase in temperature. These anthropogenic increases in temperature, may require animals to adapt to be more tolerant to heat stress. This adaptation requires heritable variation in phenotypic traits that mitigate the effect of increase in global temperature. One trait of susceptibility to heat stress is critical thermal maximum temperature (CTmax). CTmax is measured as the temperature at which an animal displays loss of equilibrium. CTmax is a phenotype that has been found to be highly variable in Fundulus heteroclitus a species of brackish killifish, however, there have been no published measures on the heritable this trait is in this species. These data are important because they will give us insight into how fish species will change as environmental temperatures increase. To quantify the heritability of CTmax, individuals with highest and lowest CT Max will be selectively bred. These selected offspring will be raised until they are mature and then tested for CTmax. Narrow sense heritability (H²) is determine by the response (ΔR) to this selective breeding (S) as defined by the breeder’s equation R=h²S. To determine heritability, this equation will be used.