Parentage of nestling red-winged blackbirds (Agelaius phoeniceus) from an eastern population was determinedusing DNA fingerprinting techniques. Of 235 nestlings surveyed, 58 had fingerprints excludingthe male, but none excluded the female tending the nest. Data on pairing status during the female's fertilizable period was available for 232 offspring; 55 (25%of 1988 nestlings, 23% of 1989 nestlings) of those were sired through extrapair copulations. Of these 55 offspring, 33 could be assigned to nearby territory holders; 16 of the remaining nestlings may have been sired by nearby males that were not captured. During both years, 44% of territorial males had more than one female nesting simultaneously on their territory. The number of extrapair fertilizations gained by males increased significandy with harem size in 1 year. Paternity (die proportion of nesdings on the territory sired by die territory holder) showed a positive but nonsignificant increase widi harem size in bodi years. There was no apparent cost in paternity for males guarding two or more fertilizable females at the same time. The broods of females that were fertilizable at die same time anodier female was setding on die same territory tended to have a greater proportion of extrapair fertilizations (0.37) than did die broods of odier females within harems (0.15). Established fertilizable females were chased significantly more by die territory owner and by extrapair males when a new female was setding. There were no associations between a male's paternity or success at gaining extrapair fertilizations and his age or color-band combination. Overall, extrapair fertilizations had litde effect on die relationship between fledgling success and harem size and appeared to have a minimal impact on die overall intensity of sexual selection on males.