Biological attack of carbonate-cemented sandstones and mudstones (Cretaceous and Eocene ages) and semi-consolidated calcareous muds (Holocene age) were studied near La Jolla, southern California. Rocks were collected from the intertidal zone to a depth of 40 m using SCUBA. The deeper samples came from Scripps and La Jolla submarine canyons. In order to learn of the borers and their preferred substrata, we studied the borers in situ as well as in aquaria, examined their borings with x-ray radiography, and analyzed the bored rocks by petrographic techniques. In preferred substrata a sequence of boring activities resulted in over half of the initialrock volume being excavated to depths of 10 cm. The major initial borers were bivalves (pholads, Lithophaga ) , which made straight smooth-walled holes up to 10 cm deep and at all angles to the rock surfaces. When abandoned, these holes were modified by other borers, and interconnected by smooth or roughhewn sinuous passageways 1 to 15 mm in diameter. The passageways were concentrated along bedding planes and in selected layers of rock, meeting at common points to form galleries. Many crustaceans and worms occupied the passageways, and several kinds of saapings and/or etchings characterized the walls. Species accomplishing this have not yet been determined. A variety of borings 1 to 5 mm in diameter have single or multiple openings into the larger passageways. Rates of biological erosion of the rock were obtained from the depth of penetration of the borers and estimated length of life spans in years. Yearly attrition is estimated to range from2 to 10 mm in some localities. Physical and chemical submarine weathering of these rocks alone seldom penetrates more thanone cm from the exposed surface. Because biological penetration is much greater, it is an important factor in shaping the seabed. Borers attack fine-grained rocks more intensely than those that are coarse-grained, perhaps accounting for the dearth of mudstones on the seabed near La Jolla, except for the steep and more rapidly eroding walls of the submarine canyons. Most rocks presently exposed on the relatively flat upper continental shelf (10 to 30 m) appear to be remnants of highly indurated sandstone strata, less susceptible to biological attack.