Since June, 1983 many corals and other reef organisms on the Caribbean and Pacific coasts of Costa Rica were observed to be bleached, due to loss of their symbiotic zooxanthellae. Some of the corals recovered by the end of October while others died. Observations, mainly at Parque Nacional Cahuita, Limón (Caribbean coast), were not conclusive as lo the causative agent for this massive death, High water temperatures, combined with other environmental stresses, could be responsible, since in June, 1983, temperatures of 33 and 35 °C were recorded at Cahuita. Apparently the boring sponge Cliona caribbea profited by the massive death of these corals.