Bleaching (loss of pigmentation) is a common response by corals to environmental stress. Changes in pigmentation and the processes that caused them were studied in Montastrea annularis during a natural bleaching event off southeast Florida. Chlorophyll c, peridinin, and diadinoxanthin levels were 35, 17, and 20 times higher in normal corals than in bleached ones. Two processes, loss of zooxanthellae from the coral and loss of pigments from the remaining zooxanthellae, contributed to bleaching. Loss of zooxanthellar pigment accounted for 72% of the decrease in Chl c. The two processes contributed about equally to reduced carotenoid levels.