The taruca (Hippocamelus antisensis) is the only deer species found permanently in rugged mountainous habitat above the tree line. I studied the social organization of this deer in relation to its reproductive cycle and habitat use in the high Andes of southern Peru. Tarucas bred seasonally. Most fawns were observed towards the end of the rainy season between February and April. Mating was most common in June, during the dry season, and antler-shedding in males occurred in September/October, at the onset of the rainy season. The deer lived in social groups and, unlike most seasonally breeding cervids, formed large mixed-sex groups nearly all year. During the birth season, however, all pregnant females segregated to form female associations. At this time, adult males were found equally in mixed-sex groups or in small all-male groups. These groups differed in their habitat use. Female groups used areas of higher elevation, steeper slopes, and greater rock-cover than either male or mixed-sex groups. I suggest that selection of more rugged and concealed habitats by lactating females is primarily an antipredator strategy to reduce risk of predation on fawns. Tarucas are compared with other social Cervidae and with their ecological counterpart: the mountain Caprinae. The social structure of Hippocamelus resembles that of wild goats (Capra spp) and other Caprinae of similar ecology but it differs from that of wild sheep (Ovis spp). / Science, Faculty of / Zoology, Department of / Graduate