Results of ecological observation on the cercariae of Echinochasmus japonicus showed that their development in and emergence from Parafossarulus striatulus as well as their activity in infecting the fish host in water were closely related to temperature and light. The number of cercariae emerged varied with the temperature of water. When the infected snails were reared at water temperature of 27-32 degrees C, the germinal cells and germ balls in the daughter radiae multiplied continually into cercariae a majority of which emerged. At water temperature below 24 degrees C, multiplication was hindered and, emergence of cercariae obviously decreased. Cercaria shedding stopped at water temperature of 14 degrees C. The reason why the rate and degree of infection with meta-cercariae of Echinochasmus japonicus in the small-sized species of fresh water fishes were higher than in the large-sized species was that the cercariae were tactic to light and so active in shallow water where they were constantly in contact with smaller fishes. Since the natural infection rate of Parafossa-rulus striatulus was found to be higher (32.2%; 37.3%) during the months of July-October than during the other months, investigations on the first intermediate host should be carried out in early autumn.