Water temperatures of Rio Grande River ranged from 0C to 29C, pH ranged from 7.4 to 8.6, total hardness varied from 86 ppm in the upper river to 194 plus or minus 6 ppm in the reservoirs. The mean critical thermal maximum (CTM) was 39.9C in fish acclimated in fresh water (10-30 ppm), 28.2C in fish acclimated in water at 85 ppm, 27.3C in fish acclimated at 171 ppm. Ion concentration of 180 ppm and 25 ppm has no effect on the CTM of the mosquito, fish, Gambusia offinis. The effect of water hardness on upper lethal temperature of the green sunfish Lepomis cyanellus acclimated at 20C and 30C and hardness levels of 30, 180, and 400 ppm indicated no significant effect of hardness on the incipient lethal temperature 35C for 20C acclimation and 40C for 30C acclimation. The relationship between incipient lethal temperature and acclimation is expressed by the equation, Log (LD50) = 1.322 + .0056 (CTM in degree C). Incipient lethal of 31C for 10C acclimated fish was obtained. There was no difference in the critical oxygen tension in active fish and nonactive fish. At 20C acclimation the mean oxygen consumption was 0.177 ml/gm/hr. (WRSIC abstract)