Development of the monogenean Heterobothrium okamotoi on the gills of tiger puffer Takifugu rubripes was monitored, based on specimens collected from experimentally infected fish kept at 20°C for 20 days. The development of the parasite, indicated by both the body length and the number of clamp pairs, was faster in the second and third weeks of infection than in the first week. The number of hematin cells, arranged on the surface of the gut to digest host blood, which was measured as an indicator of the cumulative amount of host blood taken by the parasite, increased as the surface area of the gut increased. The increase was very low, moderate and rapid in parasites with 0-1 pair of clamps, 2-3 pairs and 4 pairs, respectively. These results indicate that the growth of H. okamotoi on the gills of tiger puffer is closely associated with the increase in the number of clamp pairs and the intake of the host blood.