Poecilia sphenops was acclimated to two thermal fluctuations, each following a symmetrical and asymmetrical cycle. The critical maximum temperatures of the fish were significantly different (P < 0.001) between the thermal fluctuations: 40°C for the 20-29°C thermal cycle and exceeding 42°C for the 26-35°C cycles. Fish acclimated to the 20-29°C thermal cycle were more resistant to cold, with a critical thermal minimum of 10°C, unlike specimens of the 26-35°C regime, with a critical minimum of 11.7°C. In both cycles, the upper incipient lethal temperature interval was 38.8 to 39.5°C and the lower incipient lethal temperature interval was 10.8 to 11.8°C. Females preferred temperatures between 30 and 31°C, in all but the 26-35°C symmetric cycles. Males preferred a temperature between 23.8 and 24.2°C (symmetrical cycle). The high and low avoidance temperatures of male and female fish acclimated to the 20-29°C cycle were, respectively, 14 and 16°C, compared to the 26-35°C treatment, in which case these values were 8.4 and 11.4°C. The results indicate that P. sphenops is perfectly adapted to the marked seasons of rain and drought of their habitat.