A 10-element, 950m long, vertical hydrophone array based on fiber-optic data transmission has been developed primarily for studying the beam pattern from deep diving cetaceans emitting sonar pulses. The array elements have a configurable sampling rate and resolution with a maximum signal bandwidth of 90kHz and a maximum dynamic range of 133dB. The array has been deployed from a 14m ketch with a crew of four. In the course of the development a number of mechanical and electrical problems have been solved.