The literature on hearing in fish is reviewed. Experiments with Semotilus a. atromaculatus (Mitchill) showed perception over a range from 1 to 5,750 c.p.s. Fish from which the ears had been removed seemed to perceive frequencies from 20 to 200 c.p.s. Fish without the lateralis nerve behaved as normal fish. Normal fish could distinguish one-fifth of an octave in the range of 50 c.p.s. Trained on 50 c.p.s. they had absolute pitch for 70 c.p.s. A threshold curve over the range of 20 to 5,750 c.p.s. was established for this species.Highest sensitivity was at 280 c.p.s.; lowest at 20 c.p.s. and above 2,000 c.p.s. The movements of the fish in response to vibration stimuli were studied by means of motion pictures. The fish were able to locate the source of vibration, most likely oriented by fields of higher intensity in the experimental tank. In approaching the source the fish followed curved pathways. The relationships between length of pathway, direct distance from source of vibrations, and speed of locomotion were analysed. Measurements of sound intensity in the experimental rank indicated that intensity gradients existed along the pathways followed by the fish. Further measurements of low frequency vibrations in the water are in progress.