Laboratory aquaria experiments demonstrated that Vibrio AK-1 caused rapid and extensive bleaching of Oculina patagonica at 29°C slower and less complete bleaching at 25°C and 2O°C, and no bleaching at 16°C. The effects of temperature on the bacteria-induced bleaching experiments in aquaria were consistent with the natural bleaching of 0. patagonica in the Mediterranean Sea. In situ bleaching increased rapidly from late May to September, following the rise of surface seawater temperature, which reaches 29°C in August. During the winter, when the seawater temperature drops to 16°C, most of the bleached coral colonies recovered. Vibrio AK-1 was isolated from all bleached corals examined in the summer, but could not be isolated from healthy (unbleached) corals in the summer or from bleached and unbleached corals in the winter. The mechanism by which increased temperature causes the coral bleaching by Vibrio AK-1 is at present not clear. The bacteria grow in the laboratory relatively rapidly at 16°C (doubling time 2 h), indicating that bacterial growth is probably not the critical factor. We suggest that temperature-regulated factors affecting bacterial virulence may play a role in the bleaching process.