From April through June 1991, we tested a full-scale deterrent system for excluding alewives (Alosa pseudoharengus) from the intake of the James A. FitzPatrick Nuclear Power Plant (JAF), near Oswego, New York. This electronic system produced high-frequency broadband sound (122-128 kHz) at a source level (in decibels (dB) in reference to 1 mu Pa) of190 dB. When the system was on, the density of fish near the JAF intake decreased by as much as 96% and the number of alewives impinged on the intake screens at JAF decreased by as much as 87%. The system was effective during both day and night, and its range was greater than 80 m.