Habitat use, diet and foraging activity of male Red-breasted Mergansers (Mergus serrator) undergoing flightless wing molt along the shores of Anticosti Island, Gulf of St. Lawrence, Canada, were studied in 2005 and 2006. Red-breasted Mergansers used clear, shallow waters (≤12 m depth) that were near shore (<850 m) and over a sand-rock substrate with stands of rockweed (Fucus spp.) and kelp (Laminaria spp.). Foraging flocks used in tertidal and shallow subtidal areas (<4 m depth), whereas nonforaging flocks often used deeper subtidal waters farther offshore. Eighty percent of esophagi from 30 collected Red-breasted Mergansers contained ≥ 1 fish. Grubby (Myoxocephalus aenaeus) and sandlance (Ammodytes spp.) were in 43–53% of esophagi, and their relative abundance in an esophagus averaged 29–36%. Birds spent an average 23% of the diurnal period foraging, but foraged nearly 70% of the time when the tide was low in the morning and evening. The proportion of time that males devoted to foraging during the first half of the flightless season (18%; 19 July–12 August) was nearly half that in the second half of the season (30%; 13 August–5 September). The shallow waters along the shores of Anticosti Island are of particular importance to Red-breasted Merganser populations in eastern North America because 1) >3000 males undergo wing molt at the island and 2) these habitats are generally free of human disturbance during the flightless period.