To evaluate the feasibility of irradiation as a control measure for metagonimiasis, the metacercariae of Metagonimus yokogawai were irradiated with gamma ray, either after isolation from the sweetfish (Plecoglossus altivelis) or in situ of the fish, and their survival and development in rats were observed at 7 days post-infection. The radiation dose varied from 5 to 100 Gy for the metacercaria-irradiation group and from 5 to 500 Gy for fish-irradiation group. The results showed that the worm recovery rate from the irradiation groups decreased as the radiation dose was increased. Higher doses of radiation were required for the fish-irradiation group to obtain the same results as the metacercaria-irradiation group. The LD50 of the metacercaria-irradiation group was 4.5 Gy, whereas that of the fish-irradiation group 6.2 Gy. A few number of worms which survived until 7 days in rats were severely retarded especially in the growth of their reproductive organs, i.e., complete or partial failure in the development of testes and formation of uterine eggs. The present study revealed that irradiation of sweetfish by 200 Gy is effective to control infectivity as well as development of M. yokogawai metacercaria in rats.