Although the Japanese scallop Mizuhopecten yessoensis is the variety used in the largest production of scallops in the world, knowledge about its predators is very limited, and gastropods have not generally been recognized as a predator. This study demonstrates the impact of predatory gastropods on the Japanese scallop through a field survey and laboratory experiments. The 4-y field survey at a lagoon in northern Japan demonstrated that the frequency of scallops that have boreholes on their shells represented 11.3%–73.8% of dead individuals at 3 y old (shell height, >90 mm) that would have been harvested 7 mo later. By calculating the natural mortality of scallops after their release on the sowing—culture grounds in the lagoon, we estimated that 17.8% of the released scallops were attacked by drilling animals. This estimation, however, probably underestimates the impact of drilling predators, because the drilling frequency on small (young) individuals (shell height, <55 mm) was 10 times greater than on large (old) individuals (shell height, >99 mm) in the laboratory. Observations of the position of boreholes on the shells suggest that drilling animals accessed the left (upper) valve of the shell and drilled it randomly. By conducting another laboratory experiment on all potential drilling gastropods in the lagoon, 2 gastropod species—Pteropurpura (Ocinebrellus) inornatus (Récluz, 1851) and Boreotrophon candelabrum (Reeve, 1848) of the family Muricidae—were identified as the drilling animals. Because we also observed their behavior experimentally and there was no difference in the shapes of boreholes drilled by P. (O.) inornatus, B. candelabrum, and unidentified animals in the field, we conclude that these 2 muricid gastropods are the predators of the Japanese scallop and that the drilling frequency observed in the field reflects the predation frequency of those gastropods. The current study suggests that predation by the muricid gastropods has been overlooked in the management of Japanese scallop fisheries, even though it is highly probable that the distribution areas of predatory gastropods overlap many harvest areas of the Japanese scallop.