In vivo egg-laying by the diclidophorid monogenean Heterobothrium okamotoi was studied in artificially infected tiger puffer Takifugu rubripes. After exposed to oncomiracidia, puffers (0-year-old;n=40) were placed individually into an aquarium with recirculating sea water kept at 20°C. Parasite eggs were collected daily by filtration of drained water from the aquarium. The daily egg output per parasite was calculated by dividing the number of collected eggs by the number of parasites determined at the end of the experiment. The egg-laying, recorded for 19-33 consecutive days, fluctuated widely among individual host fish and/or parasites. The number of laid eggs increased for the first 10 days before reaching a plateau, ranging daily from less than 10 to more than 1000 eggs per parasite, with the mean daily output throughout the observation period being 51.2-362 eggs per parasite. Egg strings laid in a single output were calculated to be 2-157 cm long. The frequency of egg-laying, monitored in a fish with a single parasite, ranged from several outputs in a day to no egg-laying for up to 3 days. Egg hatching was unstable for the first few days, but subsequently the hatching rates increased to 90% or higher, irrespective of the number of eggs laid by parasites at different intensities of infection.