The effect of body size and acclimation temperature on the lethal temperatures as well as the effect of exposure to a range of experimental temperatures on the thermal tolerance of Gambusia affinis were studied. The tolerance to both upper and lower experimental temperatures were increased with increasing the body size (standard length, SL) from 1.5 to 3.5 cm. Fish acclimated to 10, 20 and 30°C showed normal heat acclimation since their LT-50 were 32.0, 36.5 and 38.5°C, respectively. The amount of increased heat tolerance for each 5°C increase in the acclimation temperature was 2°C between 10 and 20°C and 1°C between 20 and 30°C. Fish acclimated to 10, 20 and 30°C showed normal cold acclimation and their lower LT-50 were 1.0, 4.0 and 7.0°C, respectively. The amount of reduced cold tolerance temperature during cold acclimation was 1.5°C for each 5°C decrease in the acclimation temperature. The effect of exposure to changing lethal temperatures did not follow a particular pattern. In about 47% of the experiments, the fish gained heat tolerance where as in the same percentage (47%) of the experiment, the fish lost heat tolerance. In the remaining 6% of the experiments, the fish were not influenced by changing lethal temperatures.