Over an 18-month period regular counts were made of the water birds inhabiting lakes and ponds in the Lanín National Park in Argentinean Patagonia. A total of 130 bird surveys were conducted on 21 wetland areas, detecting 6,364 individuals belonging to 27 species from nine families. The most abundant and frequent species were Cloephaga poliocephala, Anas flavirostris, Fulica armillata and Fulica. leucoptera. Species richness was higher in spring and summer than in autumn and winter, involving 31 and 27 expected species, respectively. In contrast, bird abundance did not differ between season. The presence and abundance of bird species with respect to the surface area of the water body highlights the importance of certain lakes in particular. Thus, small water bodies (<100 ha) support higher numbers of birds; an average of 78.4–45.0 individuals as compared with larger lakes with an average of 18.58 birds. However, some species such as Phalacrocorax olivaceus, Podiceps major, Tachyeres patachonicus and Anas specularis were observed more at larger lakes, such differences being statistically significant. In contrast, Anas flavirostris, A. platalea and A. sibilatrix were more common in small and medium-sized lagoons. This information contributes to the development of management guidelines for the conservation of Patagonian wetlands.