Ecological and behavioral observations of a little-known high-Andean deer, the taruca (Hippocamelus antisensis), are reported from the Lake Titicaca basin, southern Peru. The deer's conservation status is considered vulnerable, with declining numbers. Tarucas were seen in groups numbering from three to 14 individuals, consisting most often of one adult male, one yearling male, and several adult and yearling females. We describe features of its pelage, size, and other physical characteristics, as well as its semi-arid, mountain grassland habitat. Also described for the first time are dominance, threat, courtship and mounting, flight, group, and comfort behavior. Several of the behavior patterns described are similar to patterns reported in North American cervids. All observations were recorded between 8 and 13 June 1975.