We monitored the nesting biology of color-banded populations of the Louisiana Waterthrush (Seiurus motacilla) at up to 25 sites across Pennsylvania from 1996–2001. Opportunistic polygyny in this species was documented for the first time during our study. Four out of 283 paired males were polygynous; one male and female were involved in a polygynous trio during two successive years. Opportunistic polygyny may have occurred primarily as a result of female territory fidelity and a locally skewed sex ratio in two cases, but appeared to be actively pursued as a mating strategy in the case of the male who was bigamous during two successive years. His aggressive territorial behavior was consistent with that reported for polygynous birds having naturally and experimentally elevated plasma levels of testosterone. The rate of nestling provisioning by this male at a failed secondary nest was substantially lower compared to his successful primary nest and compared to mean provisioning rates for monogamous males. In addition to reduced male parental care, protraction of the renesting interval following nest failure was another possible component cost of polygyny observed for one of the mates of this male. In this instance, both costs were borne by the first mated female, who was relegated to secondary status after her initial nest was depredated.