This paper describes the home range defense behavior observed when nonresident male red foxes were introduced into established home ranges of resident male-female pairs. In 12 observation periods, four intruders were introduced to each of three mated pairs which had been given three weeks to acclimate to a 4.05-hectare, fenced enclosure. The residents centered their activities around a natural den and the frequency of intruder-resident encounters decreased rapidly with increasing distance from the den. The primary home range defense was continual harassment of the intruders by the resident males through agonistic displays and chases. Physical contact was rare. Even though the resident males were dominant in less than a majority of the interactions observed, they were usually successful in displacing the intruders within a few hours. The resident females seldom interacted with the intruders.