Guanaco (Lamaguanicoe), the only native ungulate of Tierra del Fuego, uses the forests more extensively after the introduction of sheep. It is presumed that the regeneration of lenga (Nothofagus pumilio) is hampered by guanaco browsing. In this context, the diet of guanaco in Tierra del Fuego was evaluated through a microhistological analysis of feces in a livestock-free area dominated by lenga forest. Guanacos behaved largely as herbivorous grazers and exhibited seasonal variations in their grazing habits. Compared to other studies in Tierra del Fuego, which were conducted in areas where sheep and guanaco grazed together, we reported that the guanacos avoided and consumed fewer trees. These results have management implications, as the proper management of livestock could reduce the potential effects of the guanacos upon the lenga forests.