Bioerosion of coral skeletons is omnipresent on reefs and may result in coral mortality via detachment from the substrate (Gardiner 1903; Bertram 1936; MacGeachy and Stern 1976). The only general hypothesis in the literature on the dynamics of bioerosion posits that fast-growing coral species construct skeletons of low density that are easily bored by sponges, polychaetes and bivalves, i.e., the rate of attack is an inverse function of the density of the skeleton (Connell 1973). It has also been proposed that a very dense outer skeletal layer along the sides of corals, such as some Pavona (fig. 1) and Psammocora species, enhances resistance to boring organisms (Buddemeier and Kinzie 1976). However, the results presented here suggest that bioerosional damage to reef corals is positively correlated with skeletal density