Two-year-old tiger puffer Takifugu rubripes, persistently infected with the monogenean Heterobothrium okamotoi for longer than one year, were cohabitated in an aquarium with one-year-old tigerpuffer with no previous record of infection (naive fish) for 70 days. The infection level in the naive fish rose sharply, and the intensity of infection reached its highest on day 30 (estimated 16, 700 parasites on the gills per fish). In the persistently infected fish, on the other hand, no change was recorded in the infection level, which was much lower than that of the naive fish. Based on the compositions of parasite developmental stages in the two fish groups, we hypothesize that the protection mechanism found among the persistently infected fish functioned in at least three occasions : firstly when the oncomiracidium settled on the gills, secondly when the parasite developed from a worm without clamps to one with a pair of clamps, and thirdly when it migrated and grew on the branchial cavity wall. The persistently infected fish produced anti-Heterobothrium antibodies, while no antibody was detected in the naive fish. These results suggest that a persistent infection induces protection against H. okamotoi, but the factors responsible for effective prevention against reinfection remain to be clarified.