Crustaceans of the family Gonodactylidae (Order Stomatopoda) are potential predators of cultured pearl oysters in ocean-culture systems, yet they have not previously been implicated in contributing to on-farm mortality of culture stock. Improved knowledge of the threat posed by stomatopods to pearl oyster survival in ocean-culture systems, as well as possible influencing factors, is necessary to determine whether stomatopod predation merits concern and for prioritizing efforts to prevent and control predator incursions. The aims of this study were, therefore, to determine if the presence of stomatopods within culture units significantly affects cultured pearl oyster survival in an ocean-culture system, and whether predator or prey size exert influencing effects. Mortality of Akoya pearl oysters (Pinctada fucata) over a 6-wk culture period ranged from zero to 33.3% among individual culture units containing a single stomatopod (Gonodactylaceus falcatus) at the study site in north Queensland, Australia. An interaction between stomatopod size and their time at liberty within culture units, as well as an interaction between stomatopod size and pearl oyster size, explained 84.1% of the variance in pearl oyster survival among culture units. Most prominently, small stomatopods (total length: 37.0 ± 0.9 mm) caused minimal mortality of culture stock (4.6% ± 1.1%), whereas large stomatopods (total length: 69.0 ± 1.1 mm) had a significantly greater impact (20.8% ± 2.1% mortality) over the 6-wk period (P < 0.05). Given the potential for a single G. falcatus to cause up to 33.3% mortality of culture stock within a culture unit, the threat from stomatopod predation during ocean culture of pearl oysters could be significant and collection of site-specific information to assess this threat is recommended.