The underwater hearing sensitivities of two 1-year-old female harbor seals were quantified in a pool built for acoustic research, using a behavioral psychoacoustic technique. The animals were trained to respond when they detected an acoustic signal and not to respond when they did not go/no-go response. Pure tones 0.125–0.25 kHz and narrowband frequency modulated tonal signals center frequencies 0.5–100 kHz of 900 ms duration were tested. Thresholds at each frequency were measured using the up-down staircase method and defined as the stimulus level resulting in a 50% detection rate. The audiograms of the two seals did not differ statistically: both plots showed the typical mammalian U-shape, but with a wide and flat bottom. Maximum sensitivity 54 dB re 1 Pa, rms occurred at 1 kHz. The frequency range of best hearing within 10 dB of maximum sensitivity was from 0.5 to 40 kHz 61 octaves. Higher hearing thresholds indicating poorer 3 sensitivity were observed below 1 and above 40 kHz. Thresholds below 4 kHz were lower than those previously described for harbor seals, which demonstrates the importance of using quiet facilities, built specifically for acoustic research, for hearing studies in marine mammals. The results suggest that under unmasked conditions many anthropogenic noise sources and sounds from conspecifics are audible to harbor seals at greater ranges than formerly believed.