Abstract Polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) were measured in white sucker liver samples from seven pulp and paper mill sites, and three reference sites in Ontario. The mills included five bleached-kraft mills, with and without secondary treatment, and two sulfite-mechanical mills. 2,3,7,8-T4CDD and 2,3,7,8-T4CDF were the dominant congeners detected in both liver and fillet samples at all of the pulp and paper mill sites, with mean 2,3,7,8-T4CDD toxic equivalent concentrations (TEQs) as high as 124 pg g−1 in liver tissue. Concentrations of PCDDs/PCDFs in liver tissues were severalfold higher than in fillet tissue, but this difference can be accounted for by lipid normalization. Biological information, including liver weight, gonad weight, MFO activity (ethoxyresorufin-O-deethylase, EROD), and plasma sex steroids, was measured on individual fish. There was no relationship between TEQs and condition factor, gonadosomatic index, liver somatic index, or circulating plasma 11-ketotestosterone. A weak negative correlation was observed between circulating plasma testosterone and TEQs. Although there was a positive correlation (r = 0.49, p < 0.001) between MFO activity (EROD) and TEQ, one site with very low chlorine use and low TEQs had EROD activity similar to levels observed at more contaminated sites. This finding, along with recent observations that MFO activity is rapidly cleared in pulp-mill-exposed fish, casts doubt on an exclusive causal relationship between MFO activity and PCDDs/PCDFs at these sites.