The Hoary Fox Lycalopex vetulus, is a small omnivore-insectivore canid inhabiting open environments/areas of the Brazilian savannah, whose spatial organization and territoriality is still unknown. Space use and social organization of two breeding pairs with adjacent home ranges were determined through radio tracking from October 2002 to April 2003 in a mosaic of cultivated pastures and Cerrado vegetation in eastern Mato Grosso, Brazil. Home ranges were 140–299 ha in size, with individual areas of the male and female in each breeding pair overlapping extensively. After the death of both individuals of one pair, the neighboring pair progressively occupied the vacant space, expanding markedly its range into about half the area originally occupied by the previous pair. Factors driving a pair of Hoary Foxes to expand their territory into a vacant area after death of the neighboring pair were not clearly determined. Absence of territorial defence, however, could have contributed to the range shift observed. This is the first time that the response of neighboring foxes to social disruption of an adjacent pair has been documented for Hoary Foxes.