Upper thermal tolerance limits (TL50's) for early life stages of Par Pond and Florida stocks of largemouth bass (Micropterus salmoides intergrade and M. s. loridanus) were determined. Comparisons were made between 3 stages (non-feeding and feeding pro-larvae, and post-larvae), 2 initial temperatures (18° and 23° C), and instant vs. gradual (5° C/day) exposures to test temperatures between 18° and 38° C. Upper thermal tolerance limits did not differ between largemouth bass stocks, life stages, or initial temperatures. Ninety-six hour TL50 was 32.8° C for instant exposures and 34.1° C for gradual exposures (5° C/day) to high temperatures. Percent survival for 96-hour exposures to high temperatures was significantly higher for gradual exposures than instant exposures for all stocks, initial temperatures, and life stages. Calculated Maximum Weekly Average Temperature (MWAT) to protect early life stages of largemouth bass is 28.1 ° C.