Gastropods of the family Cymatiidae are a major predator of cultured pearl oysters, causing significant mortality in ocean culture systems. Improved knowledge of factors influencing cymatiid predation on pearl oysters is required to develop effective management strategies for these predators. This study determined whether size of the cymatiid, Gutturnium muricinum, is a significant factor influencing mortality of cultured Akoya pearl oysters (Pinctada fucata), and whether predation by this cymatiid on bivalve species in an ocean culture system is nonrandom. A single G. muricinum was capable of causing significant mortality in pearl oyster stock after a 6-wk culture period, with morality influenced by predator size. Oysters housed with a large (shell length: 55.0 ± 3.3 mm) G. muricinum experienced significantly greater mortality (23.3 ± 6.3%) than those in nets with a small (shell length: 37.8 ± 2.1 mm) individual (11.1 ± 0.6%). The presence of G. muricinum also had a significant impact on pearl oyster growth, but growth was not influenced by predator size. When presented with both Akoya pearl oysters and Hyotissa hyotis as potential prey items, cymatiid predation was nonrandom. Cymatiids preferentially preyed upon H. hyotis, that experienced 45.5 ± 12.4% mortality after 6 wk of ocean culture whereas, in the same nets, none of the Akoya pearl oysters were killed. Based on these results, various management strategies are discussed, and further research avenues identified.