The infection of cultured tiger puffer with the monogenean Heterobothrium okamotoi was monitored bimonthly from juvenile to marketable size for one year and a half. The infection was first detected in November, about five months after the introduction of host fish juvenils to the culture site. The parasite first attached to the gill filaments. They did not become matured until they moved to the branchial cavity wall. The life span was estimated to be less than 6 months in summer. The prevalence and intensity of infection varied with seasons and among host individuals. They sharply increased with an increase in water temperature in summer. As the intensity of infection increased, the parasite showed a higher variation in the number. The parasite sometimes showed strikingly biased distribution on the gills;it was found in significantly larger numbers on one side of gills and branchial cavity wall than on the other side, or present almost exclusively on one side. The infection did not severely affect blood parameters of host, probably because the infection level was not very high in the present case.