Very extensive bleaching of coral reef communities occurred after severe flood rains over eastern Jamaica. The loss of color was due to the mass expulsion of zooxanthellae from the tissues of Millepora, Scleractinia, Zoanthidea, and Actiniaria living in the shallow reef zones. The polyps of the bleached individuals continued to expand and feed in their normal fashion. It is believed that expulsion of the zooxanthellae was induced by contact with water of lowered osmotic pressure on the surface of the sea, rather than by sedimentation or fouling. Regeneration of the depleted zooxanthellar populations was very slow; many of the bleached colonies survived well despite the near total absence of zooxanthellae from their tissues for over 2 months.