Data provided by the Bureau of Commercial Fisheries, Technological Laboratory, Ann Arbor, Michigan, show that mercury was present in fish throughout the St. Clair River - Lake Erie System. Available information indicates that no measurable concentration of mercury was present in water supply intakes in the St. Clair - Lake Erie System. Mercury in excess of 1 mg/kg dry weight was present in sediments in the Erie, Pennsylvania, and Euclid and Cleveland, Ohio, areas and the Buffalo, Black, and Ashtabula rivers. The waters of the study area revealed no mercury except for one sample collected at mile point 13.3. The Detroit River area is the primary source of mercury in the western end of Lake Erie. This is revealed by the distribution pattern established through sediment samples.