In this work, we investigate the mechanisms of ciliate infection in the coral Acropora formosa. The results of laboratory experiments showed that healthy corals kept at 24 oC in aquaria and inoculated with cultured ciliates (3-5 cells/ml) remained healthy throughout the experimental period of 120 hours. However, corals kept at 29 oC began to experience tissue loss within three hours of the start of the experiment. There was 40%  5% tissue loss within 24 hours of the start of the experiment. The ciliates engulfed the zooxanthellae cells and eventually degraded them. The infection spreaded and killed coral pieces of 5cm in length in approximately 48 hours. Microscopic observations showed destruction of polyps, coenosarc, and the surface of the coenosteum. As the infection spread and the ciliate abundance increased in the aquaria, all coral fragments were killed. Our results show that ciliates act as opportunists feeding on zooxanthellae from physiologically compromised A. formosa. Our results suggest that temperature stress plays a role in this coral becoming susceptible to ciliate infection.