For a variety of coral species, we have studied the molecular origin of their coloration to assess the contributions of host and symbiont pigments. For the corals Catalaphyllia jardinei and an orange-emitting color morph of Lobophyllia hemprichii, the pigments belong to a particular class of green fluorescent protein-like proteins that change their color from green to red upon irradiation with approximately 400 nm light. The optical absorption and emission properties of these proteins were characterized in detail. Their spectra were found to be similar to those of phycoerythrin from cyanobacterial symbionts. To unambiguously determine the molecular origin of the coloration, we performed immunochemical studies using double diffusion in gel analysis on tissue extracts, including also a third coral species, Montastrea cavernosa, which allowed us to attribute the red fluorescent coloration to green-to-red photoconvertible fluorescent proteins. The red fluorescent proteins are localized mainly in the ectodermal tissue and contribute up to 7.0% of the total soluble cellular proteins in these species. Distinct spatial distributions of green and cyan fluorescent proteins were observed for the tissues of M. cavernosa. This observation may suggest that differently colored green fluorescent protein-like proteins have different, specific functions. In addition to green fluorescent protein-like proteins, the pigments of zooxanthellae have a strong effect on the visual appearance of the latter species.