We provide the first quantitative study on the diet of Somateria mollissima dresseri (American Eider) undergoing the flightless wing molt. Twenty-nine adult females in wing molt were collected in the St. Lawrence Estuary and Gulf of St. Lawrence, QC, Canada. We identified a total of 510 prey items comprising 6 species of molluscs, echinoderms, and crustaceans from the esophagus and proventriculus of 15 birds. Mytilus edulis (Blue Mussel) was the most important prey for eiders as it occurred in 80% of the gullets and represented nearly 95% of the total aggregate prey mass, confirming the importance of this prey item for American Eiders throughout the annual cycle. The mean length of mussels consumed by eiders in the Estuary was nearly 4 times that of mussels ingested by birds in the Gulf; factors driving this difference remain unknown. Accordingly, we suggest that future work on the foraging ecology of flightess American Eiders examine how the size of mussels consumed by birds may be influenced by the distribution of various mussel size classes available on molting grounds in the St. Lawrence Estuary and in the Gulf of St. Lawrence.