Because of their distinctive growth characteristics, widespread distribution and known capacity to concentrate contaminants, attached filamentous algae are particularly suited to application as biomonitors (Prosi, 1979). In the Great Lakes, Anderson et al. (1982) used Cladophora to assess PCB levels at Harbour Beach in western Lake Huron, while Keeney et al. (1976) compared heavy metal levels in Cladophora at two sites in eastern Lake Ontario. Since 1980, the Ontario Ministry of the Environment (MOE) has annually monitored contaminant levels in Cladophora (and on occasion in Ulothrix and Bangia) at approximately thirty nearshore sites in Lake Ontario and the Niagara River; these data will soon be available in full in MOE publications, although a partial data set is contained in a recent multi-agency report on the water duality of the Niagara River (NRTC 1984). This investigation, as part of the Great Lakes International Surveillance Plan for 1980, details the field ecology and biomonitoring potential of the dominant attached filamentous algae in nearshore Lake Huron, Georgian Bay and the North Channel.