Zooxanthellae found in Cliona viridis from littoral localities of the northwestern Mediterranean coast showed different degrees of degeneration wherever their hosts were undergoing sexual and asexual reproductive processes, whereas zooxanthellae from hosts not in reproduction were in an apparently healthy state. The zooxanthellae of the papillae and the choanosome differed significantly in diameter, probably as a consequence of the different light intensities in these areas. The total mean diameter of zooxanthellae was significantly greater in C. viridis in asexual reproduction because the symbionts increased in size just before massive degeneration. Stress before and during reproductive processes in the sponges, especially in the asexual ones, was thought to be the main cause of the massive degeneration of these dinoflagellate symbionts.