The rock—boring clam Penitella penita is an important agent of erosion in certain localities on the Pacific coast of North America. Erosive factors are separated on the basis of whether they are physical or biological. In the area studied physical factors alone remove the surface rock at the relatively slow rate of 0.5 mm per year. In rock heavily infested by P. penita the erosion rate was estimate to be in the neighborhood of 12 mm per year.