1) The author confirmed that the first intermediate host of Stamnosoma armatum is Thiara (Melania) libertina Gould on the following grounds: a) The author obtained characteristic metacercariae (encysted cercariae) which are morphologically identical with that of Stamnosoma armatum by infection-experiment to noninfected fishes (goldfish, Zacco platypus and Acheilognathus limbata) with a sort of cercariae which were found in the liver of Thiara libertina and belong to the “Cercaria B of fulvopunctata group (by Kobayashi's classification).” b) The author brought up characteristic adult worms which are morphologically identical with that of Stamnosoma armatum from the above mentioned metacercariae by feeding-experiment to noninfected dogs. 2) The above mentioned cercaria is rich in yellowish-brown pigment in the body and has a pair of eye-spots. Its body measures 0.098 (0.084-0.119) mm in length and 0.056 (0.049-0.063) mm in breadth; its tail 0.111 (0.091-0.133) in length and 0.015 (0.014-0.018) mm in breadth on an average. 3) Moreover proved the author experimentally the following facts: a) Opposite to the Nishigori's description a group of larger spines which are situated on the dorsal side of the mouth of this cercaria is not the anlage of the “head spine” of the metacercaria, but it seems highly probable that it is a sort of apparatus which functions mechanically to assist the penetration of the cercaria into the second intermediate hosts. The author termed those spines “oral spine” instead of “head spine”. b) This cercaria encysts and develops to a complete metacercaria not only in the gills but also rarely in the fins of certain fishes.