Recent mortality outbreaks in marine ecosystems have been linked to elevated seawater temperatures associated with global climate change. Acquisition of thermotolerance data is essential, not only to determine the role of temperature in mortality outbreaks, but also to predict consequences of global warming. In the NW Mediterranean region, elevated seawater temperatures during the summer periods of 1999 and 2003 caused mass mortality of the Mediterranean red coral, Corallium rubum (L. 1758). Experiments testing the upper thermal limits of this species were carried out in aquaria using samples collected from populations from 11 to 40 m depth in the Marseilles region (NW Mediterranean, France). Samples were subjected to temperature treatments between 18 and 30 °C with an exposure time of 5 and 25 days. Three biological response variables were used to evaluate effects of the treatments: coenenchyme necrosis, polyp activity and calcification rates (45Ca incorporation in calcareous skeleton). The results showed that exposure to 24 °C for 24 days caused a beginning of mortality only for the deep population, and to 25 °C for between 9 and 14 days caused mass mortality of both sample groups. The response variable results indicate that samples from the shallow population had greater thermotolerance of elevated seawater temperatures than the deep samples. The shallow samples showed greater polyp activity and higher calcification rate with a delayed necrosis response than the deep samples. These initial thermotolerance results combined with both hydrographic models and seawater temperature monitoring are the first step towards developing predictive tools for anticipating future effects of climate change in the red coral populations.