The bark structure of Nothofagus alpina (Poepp. et Endl.) Oerst., N. antarctica (G.Forster) Oerst. N. obliqua (Mirbel) Oerst., N. pumilio (Poepp. et Endl.) Krasser, N. betuloides (Mirbel) Oerst. and N. dombeyi (Mirbel) Oerst, native species from the Andean–Patagonian forests (Argentina), is described. Barks of the Nothofagus species are greyish to dark brown and have deep fissures, except that of N. obliqua. Non-collapsed secondary phloem has abundant sieve elements with scalariform compound sieve plates in oblique end walls, and with rounded to polygonal sieve areas in tangential walls. Axial parenchyma in secondary phloem is diffuse and/or arranged in discontinuous uni–triseriate tangential lines and bands. Crystalliferous axial parenchyma is present adjacent to sclerenchyma. Rays are usually homocellular, exclusively uniseriate in N. dombeyi to bi–triseriate in the other species. Fibres in secondary phloem are arranged in clusters, tangential lines and bands. Combined fibre and sclereid clusters are present in N. betuloides. Persistent rhytidomes usually include a variable proportion of bark. The present study helps to confirm the taxonomic placement of Nothofagaceae as a distinct family from Fagaceae, and supports the infrageneric classification of Nothofagus proposed by different authors.