Concentration, accumulation, and effects of polychlorinated biphenyls (PCBs) o nreproduction in tree swallows (Tachycineta bicolor) were studied at four sites in the Fox River drainage and in Green Bay, Lake Michigan, Wisconsin, USA, in 1994 and 1995. Total PCBs in eggs and newly hatched young (mean = 3.01 μg/g wet weight, years and sites combined) and 12-d-old nestlings (mean = 2.34 μg/g wet weight) at two contaminated sites (Kidney Island and Arrowhead) were higher than concentrations at two reference sites (Lake Poygan and High Cliff State Park, years and sites combined, pippers mean = 0.26 μg/g, nestlings mean = 0.01 μg/g). Concentrations of 11 PCB congeners were also higher at contaminated compared to reference sites. Polychlorinated biphenyls (PCBs) accumulated in nestlings at a higher rate (1.34–6.69 μg/d) at contaminated sites compared to reference locations (0.06–0.42 μg/d). Dichlorodiphenyldichloroethylene (DDE) was the only other organochlorine found in all samples; concentrations for all samples averaged ≤0.20 μg/g wet weight. Total PCBs and p,p′-DDE concentrations did not differ among clutches where all eggs hatched, some eggs hatched, and no eggs hatched.