This paper is a summary of observations of a biological chorus in deep waters northwest of Australia. The data were collected from underwater noise recordings made in the course of sonar performance trials at sites in the eastern Indian Ocean. Measurements are made for frequencies between 20 and 800 Hz. Chorus noise at 500 Hz, consisting of continual drumming sounds, was evident for 9–10 h in the evening, with the peak noise level occurring soon after sunset. The noise spectrum level at this peak frequency rose to 72 dBre: 1 μPa2/Hz, approximately 12 dB above the background noise level. Typical spectra and time waveforms for individual noise bursts are presented. The source of the noise is believed to be fish of the family Sciaenidae, commonly known as croakers.