A series of experiments on various factors which induce optimal in vitro excystation of the metacercariae of Metagonimus yokogawai isolated from the fish, Plecoglossus altivelis was conducted and the following results were obtained.

1) The metacercariae used in this experiment were isolated by the digestion technique therefore all of them were pretreated with the acid-pepsin solution before being applied to the various tests.

2) No excystation occurred when the metacercariae were placed in a salt solutions such as physiological saline, Tyrode solution and Veronal, Tris buffers alone or in combination.

3) The metacercariae underwent complete excystation in the trypsin and pancreatin solution in Tris buffer within an hour at 38℃. The best results were obtained in 0.8-0.9% trypsin solutions, pH 8.0-8.6 and at 38-40℃, approximately one hundred per cent excystation occurred in 40 minutes. Not only temperature but also hydrogen ion concentration played an important role causing excystation of the metacercariae in trypsin-Tris buffer solution. However, bile salts were not responsible for the excystation.

4) Agitation effect on the excystation was tested as a mechanical stimulus and it was found that the shaking stimulus accelerated the excysting mechanism, compared with the metacercariae on which it was not imposed.

It is concluded that the metacercariae pretreated in the acid pepsin solution demonstrates an essential requirement for the enzyme solution such as trypsin or pancreatin, provided with the optimum conditions of temperature and hydrogen ion concentration in excysting medium.