Critical thermal maxima (CTMAX) and minima (CTMIN), approximate upper and lower lethal temperatures, and the temperature limits of activity were determined for workers of diurnal and nocturnal species of Myrmecocystus. The CTMIN of diurnal species were between 11.2 and 12.0 C, and that of the nocturnal species was -0.4 C. All species had CTMAX between 43.3 and 48.4 C. Lower lethal temperatures of individuals of all species exposed for 2 h were below -5 C, and upper lethal temperatures were between 40 and 45 C. Diurnal species were active at 13-46 C air temperature (Ta) and 13-60 C soil surface temperature (Ts), and the nocturnal species was active at Ta and Ts of 2-30 C.