Shells of Mercenaria mercenaria, Mya arenaria and Aequipecten irradians have been subjected to abrasion in three marine environments on the Massachusetts coast. Shells were exposed to moderate surf on a gravelly sand beach for 18 hours; to moderate surf on a sand beach for 100 hours; and to sublittoral conditions on a sand bottom in 30 feet of water for one year. Abrasion is greatest on the gravelly sand beach and least in the sublittoral zone. Species with more surface area per unit weight are more rapidly reduced on a percentage basis but those with less surface area per unit weight lose more shell material in absolute terms. Grain size is of overriding importance in determining the modification of valves by abrasion. Surf action modifies pelecypod valves much more slowly than laboratory abrasion in rolling barrels. Furthermore, differential abrasion of various portions of the valve are common in surf but not in the laboratory. Exposure on a current swept sand bottom for one year results in little modification of valves, due to solution or abrasion. Valves placed in this environment gained weight due to encrusting organisms.