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We examined social group formation, movements and denning relative to other group members, home-range use, and the response to social group disruption among 188 radiocollared swift foxes (Vulpes velox) in Colorado. We found that during the initial stages of pair formation mated foxes shared dens more frequently than during the remainder of their pair-bond. The average distance between mated pairs was influenced by season and time of day, with the greatest proximity in the breeding season (727.2 m 6 42.3 SE), and during diurnal hours (463.7 6 34.7 m). Female foxes spent more time in the core area of the group home range (60.71% of locations versus 54.66% for males) and males spent more time on the range boundary (19.34% of locations versus 15.61% for females). Home-range use was influenced by season, with females spending more time in the core area in the breeding and pup-rearing seasons than in the dispersal season. Males also spent the highest proportion of their time in the core area during the breeding season, but used the boundary area more frequently than the core area in the pup-rearing season. A sex difference also was found in the response of a swift fox to the death or disappearance of its mate. All females maintained their territory in the event of mate death or disappearance; however, 50% of males emigrated from their range when the female mate died or disappeared. These differences in space use between social group members provide important insights into the territorial behavior and mating system of the swift fox and indicate that the 2 sexes likely play different roles in care of young and homerange defense.