Single polyps of Galaxea fascicularis were fixed to glass vials with underwater epoxy resin. After regeneration into microcolonies they were used for microsensor measurements of photosynthesis and calcification under different incubating temperatures. Gross photosynthesis was found highest at temperatures of 23 and 26°C (ca. 0.022 mole O₂ m^-3 s^–1), close to the ambient temperature (i.e. 26°C). At 35°C, gross photosynthesis was irreversibly inhibited as the microcolonies bleached. The net photosynthesis rapidly decreased with temperature and became negative at temperatures higher than 29°C. Profiles of O₂ and Ca²⁺ showed a strong effect of temperature on them. The concentrations of Ca²⁺ measured on the polyp surface also showed temperature dependence of Ca²⁺ uptake. In the dark and below 29°C, the surface Ca²⁺ concentration was temperature independent. During illumination, the surface Ca²⁺ concentration showed a dip at 26°C (ca. 8.7 mM), indicating maximum uptake rates at ambient temperature. However, at 32°C and higher, Ca²⁺ was slightly higher at the tissue surface than in the seawater, in both light and dark, resulting from calcium dissolution. The surface pH in light increased gradually from 8.3 to 8.6 with increasing temperature to 23°C, and thereafter remained constant to 29°C. At 32°C, the pH became slightly acidic compared with the water phase, probably due to a decrease in the uptake of CO₂ by photosynthesis. The largest difference in pH between light and dark incubations was at temperatures between 23 and 29°C (7.5-7.7 in dark versus 8.6-8.7 in light), which indicate high rates of photosynthesis and respiration in this temperature range. It is concluded that photosynthetic activity in the coral is maintained up to rather high temperatures (32°C), but corals at super-optimum temperatures (above 26°C) consume more O2 than they produce, decalcify and produce CO₂.