The metacercaria of Clinostomum complanatum excysts from fish only after activation. The present study was conducted to determine whether activation was dependent on temperature, hydrogen ion concentration or on the stomach enzyme, pepsin. The data indicate that the metacercaria is very sensitive to a high temperature such as 39°C and 42°C. Warm water at 33°C and 36°C can also induce excystment of the metacercaria but not so powerful. Following the pretreatment of a low temperature, the metacercarial excystment can occur at a room temperature. Therefore, the excystment does not necessarily occur only at the temperature above 33°C but an increase in varying temperatures may be enough to trigger excystment. Histological observations suggest that the parasite-origin proteolytic enzyme(s) play a major role in the excystment.