Low- and high-frequency sounds were tested as a means of repelling blueback herring (Alosa aestivalis) in confined-area and open-water experiments. Confined-area tests were performed by analyzing the response of blueback herring in floating net-pens to sounds differing in frequency, sound pressure levels (SPLs, given in decibels [dB] in reference to l.0 ƒÊPa), and pulse width. High frequency sounds between 110 and 140 kHz, at SPLs above 180 dB (at 1.0 m from the transducer) and at various pulse widths, elicited statistically significant (P < 0.05) avoidance responses by blueback herring. A reduced response was observed at sound frequencies of 100 and 150 kHz. Low-frequency sounds between 0.1 and 1 kHz at SPLs of 160-175 dB (at 1.0 m from the transducer) elicited only short-term startle responses. Field tests were performed at Richard B. Russell Dam (on the Savannah River at the Georgia-South Carolina border) to evaluate candidate transducers and amplifiers. In field evaluations a single high frequency transducer emitting 124.6- and 130.9-kHz sounds at an SPL of 187 or 200 dB (at 1.0m) partially repelled blueback herring that were approximately 60 m away from the transducer for periods of up to I h. These results suggest that high-frequency sound may provide an effective and inexpensive method, relative to structural measures, for reducing entrainment of blueback herring at hydropower stations.