Aquatic biota can accumulate and integrate contaminants which may be either introduced sporadically into the environment or occur in water at levels below current analytical detection limits. The determination of contaminant levels in aquatic biota in the Niagara River provides valuable information for identifying areas of contamination and sources, as well as the potential bioavailability of contaminants in the aquatic environment. Numerous persistent toxic and bioaccumulative contaminants from industry and waste disposal sites along the Niagara River, as well as their possible impact on water use, have been the subject of increasing concern in recent years. Freshwater mussels (Elliptio complanata), filamentous algae (Cladophora glomerata) and young forage fish (Notropis hudsonius) have been routinely monitored on the Niagara River since 1980. Much of the information collected led to the conclusions and recommendations of the Niagara River Toxics Committee Report (NRTC, 1984). The use of leeches (Nephelopsis obscura) as biomonitors for chlorinated phenols on the Niagara River was initiated in 1987. The 1987 biomonitoring study was designed to provide an update to previous surveys (1980, 1981, 1983, 1985) and identify the potential of other sites and tributaries to contribute contaminants to the Niagara River. Mussels and leeches collected from uncontaminated inland lakes were placed in the Niagara River in cages at specific locations, and resident Cladophora and bottom sediments were collected for the following specific objectives: identify contaminant sources or source areas requiring more detailed follow-up investigations based on uptake of contaminants in selected biomonitors, compare these results with ongoing long-term trace contaminant monitoring with indigenous species (spottail shiners, Cladophora) and identify spatial and temporal trends, and augment ongoing upstream/downstream Niagara River Toxics Management Plan programs (NRTMP, 1988) by providing information on contaminant levels in the river between Fort Erie and Niagara- on-the-Lake.