We examined prevalence of extrapair young (EPY) between first and second broods of a multibrooded passerine, Eastern Phoebe (Sayornis phoebe). During the 1998–2000 field seasons, we conducted behavioral observations to assign social parentage to offspring in nests and then conducted paternity analysis on a subset of social families where (1) the male and female maintained their pair bond for all broods within the breeding season, and (2) adults successfully fledged their first brood and subsequently attempted a second brood within the same breeding season. We used three hypervariable microsatellite loci to document the presence and frequency of EPY in first and second brood offspring from 87 social family units where all individuals had been sampled (n = 174 broods, n = 81 social males, n = 82 social females, n = 769 offspring). Although the majority of pairs were both socially and genetically monogamous within a breeding season, 17.2% (15 of 87 families) of all families had EPY in at least one brood and we found two cases of intraspecific brood parasitism. Overall, 9.2% of all broods (16 of 174 broods) contained EPY and 5.1% of all nestlings that hatched were EPY (39 of 769 nestlings). Second broods were more likely to contain EPY than first broods (4.6% first broods with EPY [4 of 87], 13.8% second broods with EPY [12 of 87]). The total number of EPY was greater in second broods than in first broods (1.0% EPY [4 of 411 nestlings] in first broods, 9.8% EPY [35 of 358 nestlings] in second broods).