Underwater hearing thresholds were measured at frequencies of 250, 500, 1000, 2000, and 4000 cps on four Aqualung equipped divers known to have normal hearing in air. In the frequency range investigated, the hearing threshold underwater is from 44 to 60 db greater than the minimum audible fields in air, and the shape of the thresholdvsfrequency curve is different. Mechanisms based upon the changed impedance relationships between the ear and the medium are invoked to explain the loss of sensitivity at 250 and 500 cps. It is suggested that the greater loss at 1000, 2000, and 4000 cps is caused by a shift in the resonant frequencies of the external meatus when it is filled with a medium having a different characteristic impedance. Evidence is presented that indicates that hearing underwater is at or near the bone conduction threshold, and this is considered as an alternate explanation for the change in shape of the threshold curve.