The monogenean Heterobothrium okamotoi, parasitizing, in clusters, the branchial cavity wall of cultured tiger puffer Takifugu rubripes, were collected together with some host tissue, in which the posterior part of the parasite body was embedded, to make in vitro observations of copulation and egg production under the stereomicroscope. Some specimens copulated with other worms within the same cluster by attaching their male copulatory organ to the ventro-lateral or dorso-lateral body of their partners. Self-fertilization was also observed. Eggs were produced at about 2-min intervals. After egg assembly in the ootype, the egg was transferred to the uterus and large numbers of eggs accumulated there. Eggs were connected, at both ends, with previous and successive ones through a continuous filament, forming a long egg string. The length of the filament connecting the eggs corresponded with the length of the duct between the ootype and the uterus. Eggs were extracted from the uterus of freshly collected worms and counted. The highest number of eggs per worm exceeded 1, 500. Based on the measurements of the egg and filament lengths and the number of eggs recorded, the longest egg string was calculated to be over 2.8 m long.