Critical thermal (CT) maxima (CTMAX)and critical thermal minima (CTMIN) were determined for minor caste workers of the red imported fire ant,Solenopsis invictaBuren. Ants were tested from polygynous colonies collected from northwestern Texas. Mean CTMAX is lower than that reported for the upper lethal temperature for short exposures and higher than that reported for the upper foraging limit. Mean CTMIN was, likewise, lower than the reported lower foraging limit. The CT values obtained at weekly intervals as colony maintenance temperatures were reduced from 29 to 9°C showed significant reductions in both CTMAX and CTMIN. The CT values averaged 40.7 (MAX) and 3.6°C (MIN). Differences between critical thermal values and those reported in previous studies of lethal and foraging temperatures are discussed.