Measured profiles of the submerged portion of the steep cliffs that ring Harrington Sound Bermuda, show that they are undercut as much as 4 to 5 m by a notch whose flat roof coincides closely with the level of extreme low tides. Nips, intertidal indentations with sloping roofs, are not present. Because of the largely subtidal position of the notch, the morphology of the undercut region, and the restricted wave energy, wave cutting is ruled out and hioerosion is proposed as the mechanism of undercutting, Biocrosion is suggested as a term for the removal of consolidated mineral or lithic substrate by the direct action of organisms. Observations of the emerged and submerged carbonate coastline of Bermuda reveal that organisms of many types alter the exposed rock surface from the upper reaches of the spray zone to depths well below sea level by processes that vary with degree of exposure, tidal range, rock type, organic community, and probably other ecological variables. A series of experiments with common rock-destroying organisms has been initiated. Experiments in the laboratory and under natural conditions show that Cliona lumpa, a boring sponge, is capable of removing as much as 6 to 7 kg of material from 1 m2 of carbonate substrate in 100 days. This corresponds to an erosion rate of calcarenite of more than 1 cm/year, If previous estimates are correct (that 90% of the boring by C. lampa is mechanical and not chemical), then 5 to 6 kg of fine-grained carbonate detritus can be supplied from 1 m2 of sponge-infected substrate in 100 days.