ABSTRACT. Laguna de los Cisnes (53°15'S) is a remote and unusual salty lagoon located in the Chilean Patagonia, declared natural monument to protect bird diversity in the area, which could also serve as a natural laboratory to monitor climate change. This study reports basic water, sediment and plankton characteristics observed during a summer (December) sampling, when the lagoon was hypersaline (51 g L-1), cold (9°C) and eutrophic, according to the high concentrations of phosphorous (0.30 ± 0.73 mg L-1), nitrate (0.66 ± 0.14 mg L-1) and chlorophyll-a (44.25 ± 2.52 μg L-1). The microalgae Spirogyra sp. and the micro-crustacean Artemia are predominating plankton. Results are discussed in the context of the climatic conditions affecting this lagoon year-round.