Little is known of the chronology of wing molt and the intensity of body molt among flightless Red-breasted Mergansers (Mergus serrator) in North America. We examined molt of 39 postbreeding males collected at Anticosti Island, Québec, Canada, from July to September 2005 and 2006. We estimated that birds were flightless for 30-33 days. Nearly 75% of males initiated the flightless period between 20 July and 3 August, and >90% were flightless from 8 to 25 August. Subadult males became flightless slightly earlier ( = 28 July; n = 7) than adult males ( = 1 August; n = 28). Most of the head and neck attained the cryptic alternate plumage before remiges were lost, consistent with observations at a nearby breeding site in eastern New Brunswick, where >75% of adult males were undergoing prealternate molt on the head region by late June. Molt intensities declined on the side, collar, and back and increased on the belly and tail throughout the period of flightlessness. Greater coverts generally molted in synchrony with new flight feathers, and most median and lesser coverts initiated replacement within 10 days of loss of remiges. This study provides evidence that the prealternate molt functions effectively to replace bright upperpart feathers with cryptic plumage prior to the flightless period. We suggest that the staggered pattern of molt among feather regions may minimize nutritional and energetic demands associated with feather replacement and changes in plumage insulation among flightless male Red-breasted Mergansers in the upper Gulf of St. Lawrence.