Helminthic parasites occur at high frequencies in waterbirds, but little is known about their effects on host population dynamics. In 2004–2005, we examined the effect of helminthic parasites on fledging success of semiprecocial American Coot (Fulica americana) chicks by experimentally dosing day-old chicks with the anthelmintic drug fenbendazole (treatments) or sterile water (controls) and measuring their survival to 40 days of age using mark—resighting analyses. In 2005, we also provided anthelmintic (treatment) or plain (control) supplemental food to incubating adults to determine whether chick survival was further influenced by parasite burdens of parents. Treated offspring had approximately half as many helminthic parasites as untreated offspring, but we could not demonstrate a reduction in parasite burdens among treated adults. Chicks that received fenbendazole in 2004 had 0.510 (85% confidence interval [CI]: 0.447–0.588) cumulative survival to 40 days of age, versus 0.387 (85% CI: 0.305–0.462) survival in untreated chicks. In 2005, offspring survival to 40 days posthatch was an additive function of both offspring and parental treatment, with survival averaging 0.578 (85% CI: 0.502–0.654) when both parents and chicks were treated, 0.461 (85% CI: 0.381–0.544) when only chicks were treated, 0.452 (85% CI: 0.375–0.533) when only adults were treated, and 0.334 (85% CI: 0.254–0.424) when neither was treated with fenbendazole. Our results provide compelling evidence that treatment with anthelmintic drugs increased fledging success and suggest that parasite burdens have important consequences for offspring survival in American Coots.