The purpose of this study was to examine the physiological responses of the coral host and their algal symbionts to elevated temperatures. Isolated polyps of the coral Galaxea fascicularis were exposed to three temperatures (24, 28 and 30°C) for 7 days and were then allowed to recover for 15 days at 24°C. Both zooxanthellae density and the maximum photosynthetic quantum yield of in hospite zooxanthellae, measured by pulse-amplitude-modulation (PAM) fluorometry, decreased after 7-day exposure to 28 and 30°C. In corals exposed to 28°C, both parameters returned to the initial level after 15-day recovery, but those exposed to 30°C died during the recovery period. A high release of healthy-looking zooxanthellae was evident in the coral exposed to 30°C, while those exposed to 24 or 28°C mainly released degraded zooxanthellae particles. The mitotic index of both retained and expelled zooxanthellae was highest at 30°C. The coral host appeared to lose the capacity of controlling algal cell division and of preferential expulsion of degraded algal cells after 7-day exposure to 30°C. The growth rate of corals was lowest at 30°C. The reduced growth rate, the low release of degraded zooxanthellae particles and high release of healthy-looking zooxanthellae, and increased mitotic index of zooxanthellae, in corals exposed to 30°C for 7 days, suggest that high temperature caused a physiological dysfunctioning at the level of host and that this change may be at least partly responsible for bleaching and the disruption of symbiosis.