Approximately 125 km of the Kalamazoo River, located in southwestern Michigan (USA), are designated as a Superfund site, with polychlorinated biphenyls (PCBs) as the contaminant of concern. Mink (Mustela vison) are a naturally occurring predator in this area and also a species of concern because of their known sensitivity to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and structurally similar compounds, such as PCBs. Four of nine mink trapped from the Kalamazoo River area of concern (KRAOC) exhibited histological evidence of a jaw lesion previously identified in ranch mink. The jaw lesion, hyperplasia of squamous epithelium in the mandible and maxilla, is known to be caused by 3,3',4,4',5-pentachlorobiphenyl (PCB 126) and TCDD. Mink trapped from an upstream reference area (Fort Custer Recreation Area [FCRA]) did not exhibit the lesion. Mean concentrations of total PCBs were 2.8 and 2.3 mg/kg wet weight in the livers of mink from the KRAOC and FCRA, respectively, and TCDD toxic equivalent (TEQ) concentrations were 0.30 and 0.11 microg/kg wet weight, respectively. Significant correlations were found between the severity of the lesion and the hepatic concentrations of total PCBs and TEQs. To our knowledge, this is the first published report of the lesion occurring in wild mink.