Currently, Montastraea annularis populations are suffering from high partial mortality rates; yet few studies have determined type, frequency and intensity of natural lesions in situ. During a year we followed natural lesions that appeared on healthy ramets of a population on a degraded reef within a Marine Protected Area in Colombia, to infer the effect on population size structure. From September 2003 to September 2004, 94% of the ramets presented lesions caused by bleaching, predation; or algae, sponge and borrower