We investigated influences of the thermal environment on patterns of body temperature (T_b), activity, and use of burrows during the active season in a population of free-living arctic ground squirrels (Spermophilus parryii). Arctic ground squirrels normally exhibited a daily 5°C range in T_b, and had higher T_b when above ground than when in burrows (P < 0.0001). This difference decreased as standard operative temperature (T_es; an index of environmental heat load) increased. Ground squirrels entered burrows more frequently on warmer compared to average or cooler days and when T_b exceeded 39°C. On cool days with heavy precipitation, ground squirrels remained almost exclusively below ground, and peaks in T_b were associated with brief aboveground forays. Time on the surface was maximal (about 80% between 0500 and 2200 h) at T_es = 17–33°C and decreased proportionately with decreasing T_es from 17°C to −2°C. Forty-six percent of variation in timing of presence above ground could be explained by a series of thermal and nonthermal variables related to environmental heat transfer. This suggests that diurnal activity patterns in this arctic environment with 24-h daylight result from a strategy that minimizes thermoregulatory costs.