The evidence that exposure to polychlorinated biphenyls (PCBs) leads to mutations is equivocal and controversial. Using multilocus DNA fingerprinting, we compared the mutation rate of tree swallows (Tachycineta bicolor) nesting at sites with high and low levels of contamination with PCBs. The upper Hudson River, USA, is highly contaminated with PCBs as a result of releases from two capacitor manufacturing plants in Hudson Falls and Fort Edward, New York, USA. Tree swallows nesting nearby have some of the highest known concentrations of PCBs in their tissues of any contemporary bird population (up to 114,000 ng PCB/g tissue). We found no difference in mutation rates between sites in New York with high PCB contamination and reference sites in Wisconsin, USA, and Ontario and Alberta, Canada, with known or presumably low levels of contamination. Thus, the mechanism behind altered reproductive behavior of tree swallows along the upper Hudson River is most likely physiological impairment, such as endocrine disruption, rather than mutation.