Poikilothermic organisms are susceptible to extreme variations in body temperature as their environment changes around them. When organisms experience chronically stressful environments, they may have to alter biological pathways to maintain homeostasis. Energetically costly responses can lead to allostatic overload and impact life functions such as growth, reproduction, and immunity. Cortisol is the major corticosteroid secreted by teleost fishes, and is a key mediator of stress-associated responses. While there has been extensive research on the physiological stress response in fishes in laboratory settings, information on how fish respond to stress in the wild is rare. Redband trout (Oncorhynchus mykiss gairdnerii) are a cold-water species that have adapted to persist in the thermally stressful environments of arid desert streams. Here, we investigate the stress response and subsequent effects on growth and size demography of wild redband trout in desert and montane ecosystems. We captured trout at 2+ years of age from 5 streams of differing temperature regimes at monthly intervals from June through October. We will measure concentrations of circulating cortisol, and relate it to habitat and morphometric data. We predict 1) there will be a seasonal variation in circulating cortisol levels associated with increased water temperature and 2) desert living fish will have adapted to the extreme environment and thus exhibit similar patterns of body size and demography as montane fish. These data will inform conservationists about the mechanisms by which organisms can adapt to the threat of increasingly warm waters associated with climatic change.