As part of the Hydro-Québec Grande-Baleine (Great Whale) hydroelectric project feasibility studies, Harlequin Duck (Histrionicus histrionicus) surveys were conducted in 1990 and 1991 in the eastern Hudson Bay and James Bay drainage basins. A total of 142 and 420 Harlequin Ducks were counted in 1991 and 1992, respectively, of which 142 (1991) and 356 (1992) were found in the area surveyed both years. Most individuals were in pairs and the overall sex-ratio did not deviate significantly from 1:1. The highest numbers of Harlequin Ducks counted over the two years were found on the Little Whale River, Des Loups-Marins Lake, and Nastapoka, À l'Eau Claire and Boutin Rivers. Highest pair densities were observed in June 1992 on rivers located in tundra and forest tundra i.e., the lower Little Whale, À l'Eau Claire and Nastapoka Rivers, and near D'Iberville Lake. In 1992, pair densities varied between 0.003 and 0.093 pair/km, depending on the watershed, and followed a latitudinal gradient. Two broods were located in 1991 and three were found during a preliminary survey conducted in 1989. Broods were located on Boutin, Nastapoka, and Great Whale Rivers, as well as along the Hudson Bay coast. The difference in the number of Harlequin Ducks found in June 1991 and 1992 may have been related to weather and methodological factors. Considering the vastness of northern Québec and the limited area surveyed during this study, we suggest that Harlequin Ducks breeding in northern Québec may well number in the thousands, and represent a very high proportion of the Greenland molting and wintering populations.