Although acoustic communication is an integral part of cichlid behaviour, its mechanism has never been identified before. In the present study, a combination of approaches was used to investigate the sound-producing mechanism of Oreochromis niloticus. Synchronisation of high-speed video data (500 frames s(-1)) and cineradiographies (250 frames s(-1)) with the sound recordings made it possible to locate the different body parts involved in sound production in territorial males. Sounds are made during a backward movement of the pelvic and pectoral girdles and a forward movement of the second pterygiophore of the anal fin. Various electrostimulation experiments, dissections and observation of histological cross-sections revealed a set of bundles (that we call the vesica longitudinalis) situated in the hypaxial musculature, ventro-laterally to the swimbladder. Contraction of these bundles should result in compression of the rib cage and also of the swimbladder, because of its close association with the serosa and ribs. Deflation of the swimbladder resulted in a reduced sound intensity.