The boring pattern of Cliona vermifera Hancock in heads of Montastrea annularis (Ellis and Solander) collected from Florida and Barbados was studied with SEM and polyester resin casting techniques. Sponge colony morphology consists of vertical lobes located dominantly within corallites, intercon- nected by horizontal filaments of varying diameters. Lobes advance within the corallite by cutting back the septa to the base of the thecal wall. Colonies often take on a "string-of-beads" morphology, with swellings localized in corallite areas. The sponge is evidently excavating the more porous corallites. Initial penetration of corallites probably occurs through the septathecal wall, by small-diameter "pioneer" filaments. These filaments are about 20 /um in diameter, and radiate outwards in all directions from lobes of the sponge colony. The normal diameter of chips of coral skeleton removed by this sponge is 40-50 Am: the diameter of chips removed inside the pioneering filaments may only be 10 um. Some rough calculations of the hydraulics of water movement within a sponge colony suggest that the peripheral areas of the colony cannot be supported by filter feeding. It is possible that the main colony supports the peripheral areas by translocation of nutrients, or that the peripheral areas are self-supporting, by means of direct absorption of dissolved organic material.