The recent development of technology for the ocean culture of juvenile giant clams (family Tridacnidae) in the South Pacific has brought attention to the predatory activities of a hitherto little studied mesogastropod genus, Cymatium (family Ranelidae). The results of research into relevant aspects of the biology of four species of Cymatium are presented together with estimates of their impact on ocean-nursery culture of Tradacna gigas and methods of predator control.