The objectives were to (1) determine the short-term thermal tolerance of several important species of aquatic insects in the Tennessee River-Reservoir system, (2) investigate the effect of acclimation temperature on tolerance, and the possibility of delayed mortality, (3) determine the most sensitive stage in the life cycle, and (4) conduct field studies on growth and emergence of selected aquatic insects within actual thermal plume areas. In laboratory tests, nymphs of Hexagenia bilineata (Ephemeroptera) and larvae of Chironomus crassicaudatus (Diptera) were highly tolerant of short term thermal shocks (six hour duration, simulating entrainment in a thermal plume and drifting to ambient). TL50 values increased from 35 exp 0 C at an acclimation temperature of 5 exp 0 C to 38 to 40 exp 0 C at an acclimation temperature of 20 to 25 exp 0 C. However, survivors of these treatments experienced higher percentages of delayed mortality compared to controls after being held for ten days to four weeks at the original acclimation temperature. The most senstitive stage found in the life cycle of H. bilineata was the egg during oviposition (time of fertilization); eggs exposed to 33 exp 0 C for 15 minutes during oviposition hatched at a significantly lower percentage than controls (ambient was 28 exp 0 C). The data generated indicate that aquatic insect species inhabiting TVA's large warm-water reservoirs are living close to their thermal maximum, and that an upper limit of 33 exp 0 C would ensure the maintenance of sizeable populations of the species studied. (ERA citation 08:047465)