Aquatic mammals use vibrissae to detect hydrodynamic stimuli over a range from 5 to 150 Hz, similar to the range detected by lateral line systems in fishes and amphibians. Manatees possess ∼5,300 vibrissae distributed over the body, innervated by ∼209,000 axons. This extensive innervation devoted to vibrissae follicles is reflected in enlarged, elaborate somatosensory regions of the gracile, cuneate, and Bischoff's brain-stem nuclei, ventrobasal thalamus, and presumptive somatosensory cortex. Our preliminary psychophysical testing indicates that in Florida and Antillean manatees the Weber fraction for detection thresholds for grating textures ranges from 0.025 to 0.14. At the lower end of this range, sensitivity is comparable to human index finger thresholds. For hydrodynamic stimuli of 5-150 Hz, detection threshold levels for manatees using facial or postfacial vibrissae were substantially lower than those reported for harbor seals and similar to reports of sensitivity for the lateral line systems of some fish. Our findings suggest that the facial and postfacial vibrissae are used to detect hydrodynamic stimuli, whereas only the facial vibrissae are used for direct contact investigation. © 2011 New York Academy of Sciences.