The intertidal area of Southeast Farallon Island, San Francisco County, California (37°42'N, 123°00'W) is riddled with numerous caves at sea level. Although the majority of these penetrate less than 8 m, a few reach lengths of over 60 m. The biota of these caves differ markedly from that occurring at corresponding tidal levels outside. While many organisms are thought to occur in caves because of reduced illumination, others appear to occupy caves because of the benefits derived from the cave's topography. The decreasing dimensions of a cave with length, combined with acute and oblique side channels, deep fractures and wall protuberances, tend to intensify and concentrate the waves that enter the cave. In these areas of considerable wave action the stalked barnacle Pollicipes polymerus Sowerby (1833) occurs in large aggregations. This barnacle is generally associated with, and is considered indicative of high energy intertidal areas along the coast of California (Ricketts & Calvin, 1968).