An investigation on the impact of predation by Ostracion immaculatus on fouling community structure in Kanayama Bay, Kii Peninsula, Japan was undertaken from April 1994 to February 1995. Caging experiments with three size groups of O. immaculatus using predator inclusion as well as exclusion treatment confirmed that predation by this fish had significant impact on the structure of fouling community. The importance of predation was manifested mainly through the influence of fish on ascidians, with fish exclusion resulting in their monopolization of the substratum within six months of exposure of the panels. The competitive exclusion by bryozoans and barnacles was observed in the absence of predators. Solitary and competitively inferior colonial forms survived better in the presence of predators. Predation by O. immaculatus was compared with the natural predation by exposing test panels outside the cage.