AIMS: To determine the relationship between yellow band disease (YBD)-associated pathogenic bacteria found in both Caribbean and Indo-Pacific reefs, and the virulence of these pathogens. YBD is one of the most significant coral diseases of the tropics. MATERIALS AND RESULTS: The consortium of four Vibrio species was isolated from YBD tissue on Indo-Pacific corals: Vibrio rotiferianus, Vibrio harveyi, Vibrio alginolyticus and Vibrio proteolyticus. This consortium affects Symbiodinium (zooxanthellae) in hospite causing symbiotic algal cell dysfunction and disorganization of algal thylakoid membrane-bound compartment from corals in both field and laboratory. Infected corals have decreased zooxanthella cell division compared with the healthy corals. Vibrios isolated from diseased Diploastrea heliopora, Fungia spp. and Herpolitha spp. of reef-building corals display pale yellow lesions, which are similar to those found on Caribbean Montastraea spp. with YBD. CONCLUSIONS: The Vibrio consortium found in YBD-infected corals in the Caribbean are close genetic relatives to those in the Indo-Pacific. The consortium directly attacks Symbiodinium spp. (zooxanthellae) within gastrodermal tissues, causing degenerated and deformed organelles, and depleted photosynthetic pigments in vitro and in situ. infected fungia spp. have decreased cell division compared with the healthy zooxanthellae: 4.9%vs 1.9%, (p > or = 0.0024), and in d. heliopora from 4.7% to 0.7% (P > or = 0.002). SIGNIFICANCE AND IMPACT OF THE STUDY: Pathogen virulence has major impacts on the survival of these important reef-building corals around the tropics.