A study of Presque Isle Bay and its tributaries was performed to evaluate present water quality and to determine cause and effect relationships between wastewater discharges and water quality. Field sampling of Presque Isle Bay, its tributaries and Erie Harbor was performed during the fall and winter of 1973 and the spring of 1974. Special wastewater studies were performed for Penn Central and for eight select industries. Garrison Run, a tributary of Presque Isle Bay, was investigated to determine sources of wastewater entering the stream. In general, water quality in Presque Isle Bay and Erie Harbor was good except for the presence of high levels of total and fecal coliform. Localized areas of degraded water quality were found in a few areas. Poor water quality was observed in the bay area around the confluence of Mill Creek and in the lake area adjacent to Hammermill Paper Company. Water quality in the three tributary streams was degraded and indicated the presence of sanitary and industrial wastewaters. Mill Creek appears to contribute the highest pollutional load to Presque Isle Bay.