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Composition and seasonal changes in guanaco diet were determined in La Payunia Protected Area through seasonal faecal sampling and food availability analysis. Diet included 32 genera of plants, mostly grasses and low shrubs. Summer use of forbs and winter use of high shrubs were more and less, respectively, than expected with regard to their availability. Seasonal alternation between grazing and browsing was detected. The herbaceous stratum provided the basic food of the guanaco; use and preference for the shrubby stratum increased when the availability of grasses and forbs decreased. Low summer selectivity may reflect the adaptability of the guanaco to the scarcity and unpredictable phenology of plants in arid environments. (C) 1996 Academic Press Limited.