Eighty to 90 % of the metacercariae of Metagonimus yokogawai excysted within 20 minutes in a trypsin solution after pretreatment with an acidfied pepsin solution. In a culture medium containing chick embryo extract, human serum and NCTC 109 in the ratio of 4: 3: 3 at 37.5 C with a gas phase of 8% CO₂ in air, gametogenesis took place, the seminal vesicle was packed with sperm, and sexual maturity finally attained, as evidenced by the appearance of 70 to 90 eggs in the uterus, but the vitellaria gave a poor Fast Red B positive reaction and the egg-shells were imperfectly formed. Addition of liver concentrate, yeast extract, a vitamin mixture, or hen-egg yolk to the medium has not given any more beneficial effects. It was found that cross fertilization may not be a necessary prerequisite to the egg-shell formation in the present species.