In a ten-year study of the huemul (Hippocamelus bisulcus) population at Torres del Paine National Park in southern Patagonia, we covered a wide range of information regarding the ecology of this endanger deer. We determined the huemul social structure, social behavior, reproductive season, and associated movements throughout the year. Huemuls were observed in the park central area of Sector Grey. When animals were located, natural marks or scars, coloring of body and face, and antler characteristics were used to identify them. We also ear tagged sixteen fawns from 2002 to 2008. Behavioral information was collected from all members with focal observations. We found established and transient social groups. Among the established we found family groups, solo female, and solo male, and among the transient we found solo female, solo male, solo yearling, pair of yearlings, and mixed groups. Family groups were observed throughout the year since they remained in the same area. We observed the first mating of marked females at about 16 months of age. We were not able to determine male age of their first mating, since they left the study site as yearlings. Females gave birth from late October to mid-November. The home range varied from 269 to 336 ha for the established family group. Huemul movements were associated with seasons and reproduction cycles, which also implies group changes. During the rut, some transient solo males tried to move into home ranges of established family groups. During the birthing season, pregnant females remained alone for short periods of time, while yearlings were temporarily expelled from their natal group but remained within the home range, thus sometimes the three members of the group were seen alone. These critical periods increased antagonism among huemuls, generating changes in their group structure. Movements associated with huemul in the park consisted of movements within the home range, seasonal movements, dispersion of the yearlings, and movements during the reproductive period.