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We studied the daily energy expenditure of free-living nonreproductive carnivorous swift foxes (Vulpes velox, average mass 2.1 kg) on shortgrass prairie in southeastern Colorado in summer and winter in relation to air temperature, daily activity pattern, movement rate, and daily movement distance. The field metabolic rate (FMR) was measured with doubly labeled water, and activity and movements were monitored by radiotelemetry. During their nighttime activity period in winter, swift foxes traveled large distances (18.5 +/- 0.6 km/d). Locomotion costs (estimated from daily movement distance in winter, using an allometric equation) accounted for at least 21% of total daily expenditure, the highest proportion reported for a mammal. During their nocturnal activity periods (winter vs. summer), swift foxes apparently traveled farther (ca. 13.0 vs. 5.7 km/night, using equal sampling intervals) and were active longer (ca. 12.9 vs. 11.3 h/night) in colder air. Nevertheless, FMR in winter (1488 kJ/d) was significantly lower than during summer (2079 kJ/d). We review available data for other free-living eutherians and show that low temperatures in winter are not necessarily associated with increases in FMR.