The average selected temperature of a sample of Gambusia affinis previously acclimated to 20°C decreased from 28-29°C to about 27"C after the fish had lived in a temperature-gradient tank for one month. The final thermal distribution of the fish varied with sex and developmental stage. Males selected lower temperatures than did adult females. The young fish, although more scattered than the adults, occurred mostly at temperatures higher than those selected by the adults. These results may indicate temperature separation of young and adult fish in nature which would reduce intraspecific predation and competition. Heat resistance of adult females taken from the 26.2°C-compartment of the temperature" gradient tank was higher than the resistance of 28.0°C acclimated fish. The temperature-gradient females either had attained a high acclimation level by temporarily venturing above 28.0°C or had accumulated high resistance by living at varying temperatures below 28.0°C.