An effort was undertaken at the Environmental Protection Agency's (EPA) Environmental Research Laboratory, Narragansett (ERL-N), Rhode Island, to evaluate the integration of in situ biological monitoring with the blue mussel, Mytilus edulis L., into EPA's Complex Effluent Toxicity Testing Program. The scope for growth (SFG) index, an instantaneous measure of energy balance in an organism, was used to assess the physiological condition of the mussels. The initial step in the program, assessing the sensitivity of M. edulis to several known toxicants in the laboratory, indicated that the SFG of the mussel is comparable in sensitivity to those of other endpoints and test species currently used for assessing receiving waters. A second step involved using the mussel in situ to assess the impact of a municipal sewage outfall on receiving water quality in Greenwich Cove, East Greenwich, Rhode Island. This was completed twice; once before the initiation of an upgrade of the facility, and once when the upgrade was about half complete. Mussels were deployed along a dilution gradient from the sewage outfall to a control station for a period of one month. Subsets of mussels were collected after a 7-day and 30-day exposure period.