This article reports a preliminary observation oh the growth of cercarial embryo of S.japonicum in defined medium cultivated one week in vitro. When extract of head-foot tissue of Oncomelania hupensis was added to the medium contained 1/2 RPMI 1640, the length of cercarial embryo increased. While liver extract of the snail was added to the same medium, the growth was inhibited.It seemed that the extract of snail liver tissue might be poisonous to the cercarial embryo.