Many herbivorous gastropods contribute to erosion of rocks by scraping algal food and substrate with their radular teeth. The daily amount of beachrock erosion by six common intertidal gastropods is measured indirectly from the dry weight of faecal pellets. Conservative estimates of the rate of erosion for species of average shell height range from 0.2 to 2.4 g/year. For a single species, the amount of erosion increases with increase in shell size. However, observations show some species are more efficient erosional agents than others. Laboratory experiments show that variations in substrate characteristics, rock hardness, and depth of algal penetration influence the amount of erosion. A possible use of data on rates of erosion by gastropods in field situations is discussed.