During the past decade, acute and chronic bleaching of tropical reef corals has occurred with increasing frequency and scale. Bleaching, i.e., the loss of pigment and the decrease in population density of symbiotic dinoflagellates (zooxanthellae), is often correlated with an increase or decrease in sea surface temperature. Because little is known of the cellular events concomitant with thermal bleaching, we have investigated the mechanism of release of zooxanthellae by the tropical sea anemone Aiptasia pulchella and the reef coral Pocillopora damicornis in response to cold and heat stress. Both species released intact host endoderm cells containing zooxanthellae. The majority of the released host cells were viable, but they soon disintegrated in the seawater leaving behind isolated zooxanthellae. The detachment and release of intact host cells suggests that thermal stress causes host cell adhesion dysfunction in these cnidarians. Knowledge of the cellular entity released by the host during bleaching provides insight into both the underlying release mechanism and the way in which natural environmental stresses evoke a bleaching response.