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(1) We examined relationships between ground squirrel activity and operative temperature (T-e) in two habitats in southwestern Idaho, USA to determine how activity patterns are tied to environmental conditions and how habitat can mediate the thermal environment. (2) Midday environmental conditions were too hot throughout much of the active season for Piute ground squirrels to remain active in a grassland habitat. (3) Ground squirrels were most active in grassland at a T-e of approximately 25 degrees C and showed a strong bimodal activity pattern during the latter half of their active season (after 15 April). (4) Suitable aboveground microclimates existed throughout must of the season in sagebrush (Artemisia tridentata) habitats and ground squirrels remained active throughout the day. (5) The availability of cooler microhabitats may make sagebrush a better habitat than grassland for Piute ground squirrels, especially during the latter portion of the active season and in hotter than normal years. (C) 1999 Elsevier Science Ltd. All rights reserved.