A hydrophone is described for determining the swimming depth of fish fitted with location-only ultrasonic transmitters. The linear array of 13 transducer elements gives omnidirectional sensitivity horizontally and a two-degree angle of acceptance vertically. The hydrophone is lowered through the water and depth of maximum horizontal signal from a transmitter is determined. Fifty-four trials in a stratified lake, with signal source at distances from 10 to 100 m and at depths from 0.5 to 20.5 m, gave a mean error of −0.46 m (standard deviation = 1.47 m).