Winter pair formation is one of the more unique aspects of waterfowl biology. Besides the dabbling ducks, relatively little is known about the chronology of pair formation and factors, such as molt chronology, which cause the between individual variation in the timing of pair formation. A small (60 - 100 birds) molting and wintering population of Harlequin ducks (Histrionicus histrionicus) was studied from June to November 1995 to assess the molting and pairing chronology of this population. Males returned from the breeding grounds in June and July, and immediately began the pre-basic molt. Most males were in the basic plumage and flightless by late July through August. The pre- alternate molt began in September and most males were back in full alternate plumage by the beginning of October. Courtship and pairing began immediately after the finish of the pre-alternate molt. At the time some males were in alternate plumage and others were still molting, males in alternate plumage exhibited courtship behavio r more frequently. Through August and September, the sexes tended to segregate into same sex groupings and males were clumped into large groups, possibly interacting and establishing a dominance hierarchy. In October, mixed sex groups were more prevalent and all the ducks were much more dispersed. Pair formation peaked at two different periods during the non-breeding season. Adult females re-established previous pair bonds with males in the fall and young females, presumably pairing for the first time, p aired in the spring. As males are molting immediately after arrival on the molting grounds and begin to court females immediately after they have finished their molt, it is very likely that there is strong sexual selection for early pairing in Harlequin Ducks, and probably most other sea ducks.