Leeches were previously shown to have unusually high bioaccumulation capacities for chlorophenols (CPs) and have, therefore, been recommended as biomonitors for CP pollution. To determine whether leeches could survive on-site exposures in cages and to compare leeches and mussels as biomonitors for CPs, two field studies were conducted. CP residues in leeches ( Nephelopsis obscura) exposed for 3 weeks above and below a pulp mill complex on the Rainy River, Ontario, were compared with concentrations in resident mussels, while leeches and mussels ( Elliptio complanata) were directly compared to exposing them simultaneously in cages near a wood preserving plant on Thunder Bay Harbour. Survival of caged leeches in the Rainy River was excellent (77–87%), provided conductivity/alkalinity was within the tolerance range of the species. Leeches as far as 100 km downstream of the mills accumulated elevated levels of CPs (21–121 ng/g vs. 4–5 ng/g pre-exposure), with proportions of the various congeners reflecting those in mill effluent. Concentrations of CPs in mussels (<0.5–3.5 ng/g) were 1–2 orders of magnitude lower than in leeches. Leeches in Thunder Bay Harbour accumulated four CPs, with concentrations of the dominant pentachlorophenol (PCP) ranging from 817–5,300 ng/g near the plant to 275–375 ng/g at 100 m, 55–85 ng/g at 300 m, and < 50 ng/g at 600 m beyond the discharges. In contrast, CPs were never detected in mussels from any site. Bioassays conducted on harbor sediment using fathead minnows ( Pimephales promelas), burrowing mayflies ( Hexagenia limbata), and leeches demonstrated that uptake and toxicity decreased with increasing distance from the plant. At all sites, PCP residues were similar in field-and lab-exposed leeches. This study demonstrated that leeches are far superior to mussels as biomonitors for CPs, and that leeches can be successfully caged and used in routine monitoring where CP pollution is suspected.