Coral health was assessed by monitoring the photosynthetic activity of their zooxanthellae using the fast chlorophyll a fluorescence rise 0-J-I-P (Strasser et al. 2000). This type of measurement has the advantage of being non-invasive. Three species of recently collected Anthozoa were being studied: two species of Actinodiscus from Bali (Indonesia) and one colony of Porites porites from St Lucia (Lesser Antilles). The collected data were analysed according to the JIP-test (Strasser et al. 2000). Clear circadian rhythms were found in all calculated fluorescence parameters. The measurements are sensitive to random noise caused by for example movements of the polyps. We chose to study the maximum quantum efficiency of PSII (ϕPo) for its relative insensivity to this noise and the performance index (PI_ABS) as a reflection of all photosynthetic processes in PSII. The peaks and troughs of the circadian oscillations did not coincide with the rhythm of the ambient light in the aquarium. Long-term experiments with light cycles are reported.