Gymnophalloides seoi is a human intestinal trematode prevalent on southwestern islands in Korea. In the present study, we investigated whether G. seoi metacercariae can grow and develop into adults by in vitro cultivation. The metacercariae were obtained from naturally infected oysters, and cultured in vitro for 5 days under three conditions; 37℃/5% CO₂, 41℃/8% CO₂, or 41℃/5% CO₂, in NCTC 109 complete media containing 20% FBS and 1% antibiotics-antimycotics. The degree of worm growth and development was compared with that grown in vivo of C3H mice. The length of the worms cultivated in vitro was 200-300 µm not significantly different from metacercariae, whereas the length of the worms recovered from C3H mice was significantly larger, 300-400 µm. The worms produced eggs when grown in C3H mice or cultured in vitro for 2 days under 41℃/8% CO₂ or 41℃/5% CO₂, but not when cultured under 37℃/5% CO₂. Among the in vitro conditions, 41℃/5% CO₂ was best for egg production, although the number of eggs was about half of worms obtained from C3H mice. In conclusion, in vitro cultivation of G. seoi metacercariae into egg-producing adults was partially successful under culture conditions of 41℃/5% CO₂ or 41℃/8% CO₂.