In situ photosynthetic fluorescence of the zooxanthellate Mediterranean coral Cladocora caespitosa (L.) was measured seasonally on colonies from 5 to 27 m depth using an INF-300 Integrating Natural Fluorometer (Biospherical Instrument Inc.). This oceanographic instrument, used to measure the in vivo phytoplankton chlorophyll a (Chl a) fluorescence, was adapted to record the natural fluorescence of C. caespitosa by SCUBA divers. The resulting curves of natural fluorescence of Chl a vs photosynthetically active radiation (PAR 400-700 nm) showed that: (1) natural fluorescence was limited by light availability in both deep and shallow colonies in all seasons; (2) photosynthesis occurred in C. caespitosa also in winter, when temperature is low and seawater turbidity contributes significantly to PAR attenuation; and (3) the efficiency of the Chl a fluorescence increased from summer to winter. This last finding outlines the winter coupling between zooxanthellae activity and calcification processes and is consistent with the formation of the high density band in the coral skeleton.