Gymnophalloides seoi has drawn medical attentions since the discovery of the first human case and a highly endemic area on a southwestern coastal island of Shinangun, Korea. Marine bivalves especially oysters were strongly suspected as the source of infection. In this study the oysters, Crassostrea gigas, naturally produced from the endemic area were examined whether they contain gymnophallid metacercariae. All of 50 oysters examined were infected with the metacercariae of a gymnophallid, with the metacercarial density per oyster of 610 on average (2-4, 792 in range). Later they were identified as G. seoi by obtaining adult worms from experimental mice. The metacercariae were unencysted, and firmly attached on the mantle surface of the oysters with their oral sucker. In sectioned specimens they were equipped with the ventral pit, a peculiar organ of the genus Gymnophalloides, and non-muscular genital pore which was connected dorsally to the seminal vesicle. The seminal vesicle was in a great majority mono-sac. By this study, it has been confirmed that the oyster is a 2nd intermediate host of G. seoi as well as the major source of human infection with this fluke.