AbstractThe water‐soluble arsenic compounds in five species of clams – Butter clam (Saxidomus giganteus), Horse clam (Schizothoerus nuttalli), Soft‐shelled clam (Mya arenaria), Native littleneck clam (Protothaca staminea), and Manila clam (Venerupis japonica) – are described. Varying amounts of arsenobetaine and tetramethylarsonium ion are the major arsenicals found in all species. Butter clams show the presence of a third compound which appears to be trimethylarsine oxide. Small amounts of as‐yet‐unidentified arsenicals can also