Using critical thermal maximum tests (CTMax), we examined the relationship between upper temperature tolerances and fish size (fry-adult/subadult lengths) of rainbow trout Oncorhynchus mykiss (41-200 mm total length [TL]); Apache trout O. gilae apache (40-220 mm TL); largemouth bass Micropterus salmoides (72-266 mm TL); Nile tilapia Oreochromis niloticus (35-206 mm TL); channel catfish Ictalurus punctatus (62-264 mm TL) and Rio Grande cutthroat trout O. clarki virginalis (36-181mm TL). Rainbow trout and Apache trout were acclimated at 18°C; Rio Grande cutthroat trout were acclimated at 14°C; and Nile tilapia, largemouth bass and channel catfish were acclimated at 25°C, all for 14 d. Critical thermal maximum temperatures (CTMax) were estimated and data was analyzed using simple linear regression. There was no significant relationship (P > 0.05) between thermal tolerance and length for Nile tilapia (P = 0.33), channel catfish (P = 0.55), rainbow trout (P = 0.76), or largemouth bass (P = 0.93) for the length ranges we tested. There was a significant negative relationship between thermal tolerance and length for Rio Grande cutthroat trout (R2 = 0.412, P < 0.001), and Apache trout (R2 = 0.1374, P = 0.028); however, the difference was less than 1°C across all lengths of Apache trout tested and about 1.3°C across all lengths of Rio Grande cutthroat trout tested. Because there was either no, or at most, a slight relationship between upper thermal tolerance and size; management and research decisions based on upper thermal tolerance should be similar for the range of sizes within each species we tested. However, the different sizes we tested only encompassed life stages ranging from fry to adult/sub-adult so thermal tolerance of eggs, alevins and larger adults should also be considered before making management decisions affecting an entire species. Results are in press with the Transactions of the American Fisheries Society and were published in an MS thesis.