The behavioral responses of fishes, specifically preference and avoidance reactions, to temperature are described using a general graphical model based on a broad overview of the literature. A quantitative measure of eurythermality, based on the area of the thermal preference zone, is presented which offers a more meaningful estimate of the optimal thermal requirements of the species than one based on lethality alone. The authors also emphasize the importance of considering the preference-avoidance responses of fishes when setting ambient water quality criteria for temperature or when estimating the potential environmental impact resulting from changes in the thermal regime of an aquatic system.