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As poikilotherms, fish health is compromised by exposure to elevated temperatures (e.g. climate change-related warming, anthropogenic thermal pollution, and/or hatchery processes). While fish thermotolerance has been demonstrated to be plastic, the downstream impacts of early life-stage high temperature exposure are not known. In the present study, we investigated the thermotolerance of rainbow trout (Oncorhynchus mykiss) fry 2months after being exposed to elevated temperature (22 degrees C) for 96h. Exposed fry demonstrated a reduced critical thermal maxima (CTmax) in comparison to control fish. Using the RNase H-dependent quantitative PCR method, expression of rainbow trout hsp70 isoforms was determined immediately after the acute thermal stress and immediately following the thermotolerance trials. The lowered CTmax was associated with a reduced ability to upregulate the hsp70b gene during the thermotolerance trials, whereas no changes in hsp70a were observed. Overall, these results indicate that exposure to thermal stress in early life-stages of rainbow trout can have negative effects on future physiological function.