We studied the nesting of the Louisiana Waterthrush (Seiurus motacilla) in western Pennsylvania from 1996 to 2008. Eight of 143 pairs (5.6%) that successfully fledged young from their first nest attempted a second brood, the first cases of double brooding reported for this species. Given the potential for double brooding to increase annual productivity, its infrequency suggests strong costs offsetting this life-history strategy. Molting is a late-season constraint that may influence the occurrence of double brooding in this migratory species. The waterthrushes' nesting season extended from 14 April to 30 July. A single nesting cycle, from clutch initiation through post-fledging care, encompassed 48–55 days. The mean date of clutch initiation for first attempts of single-brooded pairs was 5 May, with at least two thirds of pairs renesting if their first nest attempt failed; the mean date of initiation of second clutches was 25 May. The mean dates of first and second clutches of eight double-brooded pairs were 29 April and 4 June, respectively. The average date of onset of molt was 26 June, and molt was completed in about 40 days, or by early August in most birds. Double-brooded and very late-renesting single-brooded birds delayed molt by up to 3 weeks later than average, and their molt and breeding overlapped by up to 4 weeks. Delayed molt and migration associated with late nesting may be an especially significant cost of double brooding for the Louisiana Waterthrush because it is among the earliest migrants returning to its wintering grounds, where the birds compete strongly for optimum territories.