Data on the occurrence and habitat use of giant otters Pteronura brasiliensis in the 4,437 km2 of Balbina hydroelectric lake in central Amazonia, Brazil, were collected from September 2001 to December 2005. Twenty-nine groups of giant otters were recorded in an area corresponding to 10% of the total reservoir. Despite the fact that it is a hydroelectric lake, the water level can vary annually by up to 3.5 m with the Amazonian seasonal flood pulse. A total of 210 dens, communal latrines and campsites of giant otters were located. An average of 37% of the dens observed were classified as in use, and the species was recorded in the lake throughout the year. Giant otters started their daily activities between 05.23 and 08.05. The average height of den openings was 28.77 ± SD 10.00 cm, with an average width of 56.11 ± SD 19.17 cm. Births in Balbina occur mainly during the period of high and receding water level, as reported for giant otters in other Amazonian areas. There are at least two conditions that enable the species to inhabit hydroelectric reservoirs: (1) presence of the species in the area before dam construction and (2) absence or low numbers of human communities around the reservoir. The potential use of hydroelectric lakes for the conservation of the giant otter in the Amazon is discussed.