ABSTRACT Silver catfish, Rhamdia quelen, is widely distributed in rivers and lakes of Brazil and used in fish culture. The objective of this study was to determine the lower and the upper lethal temperatures for fingerlings of this species. Fishes were acclimated to temperatures of 16, 21, 26, and 31°C for 22 days. Experiments increasing or decreasing water temperature of the tanks were carried out to obtain 0 and 100% mortality. The mean lethal temperature (LT50) was calculated by nonlinear, sigmoid regression. The lower LT50 values were 7.71, 9.87, 10.97, and 14.22°C, and the upper values were 33.5, 33.55, 34.5, and 34.5°C for acclimation temperatures of 16, 21, 26, and 31°C, respectively. The area of the thermal tolerance polygon (expressed as °C2) for fingerlings of silver catfish was 804, indicating that this species is eurythermal.