Acoustic signals are widely used by fish such as pomacentrids, to communicate during courtship and agonistic activities. Adult Stegastes lividus holds a permanent territory in a coral reef environment. Previous studies suggested that S. lividus seems to produce clicking sounds when intruders close to its territory. This study aimed to describe (1) the sounds produced by this species; (2) the behavioral pattern associated with these acoustic signals; (3) vocal activity of diurnal variation. The study site was near the water inlet of the Third Nuclear Power Plant in southern Taiwan (120¡C45¡¬E , 21¡C57¡¬N). 24-hour recordings at the habitat of S. lividus at 5-6 m and at 2-3 m were made in February, March and April, 2005. The synchronous video-audio recordings were also made at the colonies of S. lividus at 2-3 m. The observations showed that S. lividus produced clicking sounds for territorial defense (e.g. aggressive behavior). These sounds were composed of a variable number of pulses that had a dominant frequency at 200 to 400 Hz. Stegastes lividus produced different sound types to defend against conspecifics or heterospecifics. Although the acoustic signals were emitted throughout all day, more sounds were recorded during the noon, fewer sounds at night, and a small peak presented at sunset. Therefore, I suggested the acoustic signals of S. lividus may be also serves as species recognition. The fish may be a diurnal fish, and its social interaction peaks at noon.