Guanaco (Lama guanicoe) is one of the most characteristic species of Tierra del Fuego province. The damage produced by guanacos in the regeneration of lenga (Nothofagus pumilio) in ecotonal forests has been documented. The aim of this study were to determine the diet composition of guanaco in a forest area that includes a portion of wetlands and to estimated seasonal presence and frequency of Nothofagus spp. in the diet. The guanaco diet was assessed by fecal micro-histological analyses. The proportion of species consumed by guanaco showed significant differences throughout the year. Caespitose grasses and rhizomatous grasses represent 52% of the annual diet of guanaco. Festuca spp., Poa spp., Carex spp. and Uncinia lechleriana are, within their respective groups, the most consumed items. Consideration of non epidermal tissue as phellem is essential to identify Nothofagus spp. in the diet of guanaco in Tierra del Fuego. The presence of Nothofagus spp. is constant and abundant throughout the year and represents the 29 % of the annual guanaco diet in Tierra del Fuego. Guanaco consumed saplings of Nothofagus spp. with presence of leaves and/or buds at different times of the year. The major consumption of Nothofagus spp. seedlings by guanaco could adversely affect the development of regeneration, and this aspect should be considered in the management and implementation of forestry or livestock activities in the forest.