Group size, activity and spatial segregation of sympatric guanacos and vicunas have been investigated in the wild in the Catamarca Andes, Argentina. A total of 65 groups of vicunas and 32 of guanacos was sighted. The size of vicuna and guanaco groups (mean ± SD: 4.8 ± 4.5 individuals and 3.5 ± 3.6 individuals respectively) fell within the range reported by other studies. The occurrence of solitary guanacos was unusually high. Spatial overlap between vicunas and guanacos was wide and occurred from 4000 to 4200 m altitude. Possibly, the two species attained segregation through the use of areas differing in slope gradient. Most guanacos and vicunas were foraging when spotted. The proportions of individuals engaged in foraging, resting and walking were, however, different between the two species, the percentage of guanacos grazing (94%) being particularly high in comparison with vicunas (69%). Vicuna activity pattern varied throughout the day. They were on the move mainly just after midday and rested mainly late in the afternoon. Guanaco group size and pattern of activity showed more variations from other studies than those of vicuna. These differences may be related to the high altitude. In such an area, which lies at the upper limit of their altitudinal range, guanacos may have problems in meeting their energy demands, which are higher than those of vicunas. Competition between the two species may occur in sympatric areas.