One-year old tiger puffer, Takifugu rubripes, were infected with the monogenean, Heterobothrium okamotoi, by exposing oncomiracidia of the parasite. Maximal intensity of infection on the gills and branchial cavity wall was 79 and 76, respectively. Immature parasites were found on the gill filaments of the host for the first 5-7 weeks until they grew up to 4.3 mm in average body length. Then, they moved to the branchial cavity wall and became matured. Approximatly 40% of immature parasites were estimated to fail to move successfully from the gills to the branchial cavity wall. Adult body size ranged from 7.0 mm to 24.3 mm long. Copulation and egg laying were first observed 49 days post-exposure. Estimated life span was within 4 months post-exposure.Parasites sometimes showed biased distribution either on the right or on the left side of the gills or the branchial cavity wall. A significant difference was also noticed in the growth between parasites on one side of the gills or branchial cavity wall and on the other. Infected fish showed anemia, as indicated by the decreased hematocrit value and hemoglobin content with the growth of parasites.