The sensitivity of the vibrissae of three Harbour seals (Phoca vitulina) to vibrations ranging between 50 and 1000 Hz was estimated using standard psychophysical techniques. Their vibrissae are relatively insensitive at the lower frequencies tested, however, vibrations which displaced the animal's whiskers by less than one micron were perceptible at 1000 Hz. Further preliminary tests indicated that the animals can use their vibrissae to perceive vibrations beyond this frequency. It is hypothesized that marine mammals can use their whiskers to detect the large amplitude water displacements propagated by objects vibrating at low frequencies. This ability would permit the animal to determine various properties of objects moving underwater without having to touch them.