ABSTRACT: The causative agent of bleaching in the coral Oculina patagonica in the Mediterranean Sea is Vibrio shiloi. The bacterium becomes more virulent with increasing seawater temperature, resulting in extensive coral bleaching (80 to90%) every summer in 1 to 6 m depth. However, colonies in shallow water (0 to 80 cm depth) showed negligible bleaching, despite being exposed to higher temperatures (ca 2°C) than the deeper water corals. Fragments transplanted from 4 m depth to a shallowreef flat (< 30 cm depth) in May showed no bleaching during the summer months, whereas intact colonies at 4 m and fragments transplanted to that depth from the reef flat underwent extensive bleaching. The concentration of V. shiloi was very high(>2 x 10⁴ cm^-2) in bleached fragments at 4 m, whereas in non-bleached fragments from the reef flat, V. shiloi could not be detected. When O. patagonica was infected with V. shiloiin laboratory aquaria and subsequently exposed to sunlight, the intracellular bacteria were rapidly killed, aborting the infection and preventing bleaching. However, when the infected corals were protected from ultraviolet (UV) light, the intracellularV. shiloi multiplied and the coral bleached. We present here for the first time evidence for prevention of coral bacterial bleaching by UV radiation (UVR).