A vertical, water-filled steel pipe, 3 m long with 0.63-cm-thick walls and 15-cm internal diameter was used to determine the biophysical basis of underwater hearing sensitivity in Rana catesbeiana. A speaker inside the base produced standing waves. Pressure and particle motion were measured with a hydrophone and geophone, respectively, and were 90° out of phase along the length of the tube. Microphonic responses were recorded from the inner ear of frogs lowered through pressure and particle motion maxima and minima. The air-filled lungs of whole frogs produced distortions of the sound field. Preparations of heads with only an air-filled middle ear produced little distortion and showed clear pressure tracking at sound intensities 10–20 dB above hearing thresholds from 0.2 to 3.0 kHz. Filling the middle ear with water decreased or abolished microphonic responses. Severing the stapes reduced responses except at certain frequencies below about 1 kHz which varied with body size and likely represent resonant frequencies of the middle ear cavity. We conclude that Rana catesbeiana responds to underwater sound pressure from about 0.2 to 3.0 kHz with the middle ear cavity responsible for pressure transduction. [Work supported by NSF and NIH.]