The antiparasitic effect of in-feed caprylic acid against the monogenean Heterobothrium okamotoi that infects cultured tiger puffers Takifugu rubripes was examined at two commercial farms. One experiment was made at a commercial farm in Oita Prefecture for 15 days (Experiment I) and the other experiment in Nagasaki Prefecture for 17 days (Experiment II). Two net pens were used at each farm to compare treated and untreated fish. In each site, the group treated with caprylic acid was fed daily a diet of pellets adsorbed with 5 g caprylic acid/kg diet at 100 mg/kg B.W./day and the control was fed the same diet with no caprylic acid. At the end of Experiment I, the number of immature H. okamotoi on the gills of the group treated with caprylic acid was significantly fewer than that of the control. In Experiment II, many fish died, probably caused by heavy infection of Cryptocaryon irritans and the number of immature H. okamotoi on the gills of the group treated with caprylic acid was significantly fewer than that of the control on the final day. Although C. irritans infection could have influenced, it is possible to think that in-feed caprylic acid also had an antiparasitic effect against H. okamotoi, even with complications of C. irritans infections. These results suggest that in-feed caprylic acid had an antiparasitic effect against H. okamotoi on a commercial scale and can be used to control H. okamotoi infections in aquaculture of tiger puffer.