We studied social organization and behavior of arctic foxes (Alopex lagopus) in 3 adjacent territories from 1988 to 1995 in central Norway, where the major rodent prey were cyclic. Twenty-five foxes were equipped with radiotransmitters and several other individuals could be recognized visually. Paired adult foxes and their cubs had home ranges that overlapped more with each other than with neighboring foxes (37% versus 2.9%) and therefore seemed to be territorial. Family composition varied from monogamous pairs, to pairs with additional adults, to an example with 2 reproducing females and 1 male in the same den. Presence of additional family members was independent of the state of the rodent cycle and the reproductive state of the breeding pair. Additional family members contributed only 2% of the food provided to young pups and were therefore not regarded as true helpers. Breeding adults remained resident within the same territory even in years with low abundance of rodents, when they did not breed. All pups left their natal territory by 6 months of age, although some subsequently returned to their natal range or one of the adjacent dens.