Selection of preferred vega habitat by family group and solo male guanacos was studied on 14 sites in Torres del Paine National Park, Chile during 7 2-week sampling intervals from 1 November to 15 February. Vegas were chosen such that 7 were dominated by family group territorial males and 7 were defended by solo territorial males. Of 32 territorial males observed on the 14 sites, 17 were family group territorial and 15 were solo males. Average group sizes ranged from 1 for solo males to 5 for family group males. Forage related and site physical characteristics were quantified and analysis of variance was used to test for differences between family group and solo male vega sites. Correlation analysis was used to examine potential relationships between vega characteristics and the degree of family group use. Vega physical and forage characteristics were both related to number of females, and may both impact guanaco populations through their influences upon predation of guanacos by pumas and guanaco foraging efficiency, respectively. Female guanacos probably selected vega habitat over all other available types based upon forage nutritional characteristics. However, female selection among particular vegas was most likely mediated by both site physical and forage characteristics. The characteristics most related to female use of vegas were grazing use during the previous month, vega surface area, and vega perimeter. Other variables appreciably related to female use were unit standing crop, unit productivity, forage moisture content, meters of periphery possessing slopes > 20%, PCI (an index of risk to predation), unit standing DDM, and plant maturity. Percent occurrence of Eleocharis pachycarpa, a small sedge, was also weakly related to female use. No significant relationships were established between use of vegas by females and absolute or percent meters of cover or slope located at a vega periphery. Additionally, no relationships were observed between female use and percent peripheral slope per unit area, vega total productivity of caged vegetation, vega total grazed standing crop, plant height, forage availability index (plant height x vega area), percent crude protein, standing crop crude protein, percent dry matter, total standing dry matter, and species composition of Carex gayana, Holcus lanatus, Agrostis capillaris, Hordeum comosum, or Potentilla anserina. Predation and forage/nutrition related factors must be investigated simultaneously in trying to understand tradeoffs that ungulates (as prey) must deal with to survive in a complex and interactive ecological environment.