Paired toadfish Opsanus tau sonic muscles, situated on the lateral surfaces of the swimbladder, commonly contract around 200 Hz to produce the boatwhistle advertisement call. To determine how sounds are produced, we electrically stimulated the sonic nerve and monitored contraction of the sonic muscles with emg electrodes, a laser vibrometer to measure bladder displacement, and a microphone to relate movement to sound production. The muscle fibres are curved like the human diaphragm, and a twitch generates complicated bladder movement suggesting a quadrapole source: the sides push in, increasing internal pressure that pushes the bottom of the bladder out. The acoustic waveform mirrors this movement; a rapid peak of negative pressure (sides in) is followed by a larger positive peak (bottom out). The peak of acoustic pressure occurs in the middle of contraction when velocity is maximal and acceleration is zero. Peak sound amplitude and bladder velocity increase to almost 200 Hz, indicating a specialisation for the boatwhistle, are robust at 400 Hz, and do not completely tetanise At 500 Hz. The bladder produces more acoustic pressure per displacement at higher frequencies, indicating that peak amplitude at boatwhistle frequencies is caused by optimal muscle contraction and not bladder resonance. The swimbladder is an inefficient sound source, and we suggest rapid contraction evolved to produce louder sounds.