Prof. Dr. M. Suzuki hold the opinion that the so-called Metagonimus yokogawai includes two different species, the one containing the smaller eggs in its uterus and the other the larger ones. He named the latter species temporarily “Large egg type of Metagonimus”, and differentiated it from the former (proper Metagonimus yokogawai). Under guidance of the professor, the author made the investigations on the morphologie and life-history of the above-mentioned two species of Metagonimus, and came to the following results: 1) Many adult worms of Metagonimus yokogawai were found in the intestines of a certain animals fed on Plecoglossus altivelis. 2) Many adult worms of so-called “Large egg type of Metagonimus” were found in the similar animals fed on each one of the following fishes: Carassius auratus, Cyprinus carpio, pseudorasbora parva, Parasilurus asotus and Mugil cephalus. 3) The so-called “Large egg type of Metagonimus” differs from the other species of Metagonimus ever described such as M. ovatus, M. romanicus, and M. dobrogensis in regards mainly to the following points: a) The posterior ends of the both vitellaria of this new species are located very closely each other in the posterior end of the body. b) The eggs of this new species are larger than those of the other species of Metagonimus [0.033 (0.030-0.036): 0.021 (0.018-0.023) mm]. 4) Having examined the structure of the cercariae which are found in the liver of Thiára (Melania) libertina Gould and belong to the “Cercaria A of Cercaria fulvopunctata group (by Kobayashi's classification)”, the author found that they are not of single species but include three different species of cercaria. The author gave the name “Cercaria A(1) (Taf. I), Cercaria A(2) (Taf. VIII) and Cercaria A(3) (Taf. IV)” to these three species of cercaria respectively. The morphological differences in these three species are chiefly those of the size of the body and tail, the clor of the body, the size and color of the eyespots and the number of the oral spines. 5) As to the life-history of these three species of cercaria, the author confirmed experimentally the following facts: a) The “Cercaria A1” encysts itself in the gold-fish or Carassius auratus readily, and the encysted larvae ingested by a certain animals further deevrop into adult worms of so-called “Large egg type of Metagonimus”, b) The “Cercaria A(3)” encysts itself in the fins and scales of Plecoglossus altivelis readily, and the encysted larvae develop into adult worms of Metagonimus yokogawai in the similar animals. c) It was extremely difficult to induce the infection of the “Cercaria A1” to the Plecoglossus altivelis, as well the infection of the “Cercaria A(3)” to the gold-fish or Carassius auratus. d) The “Cercaria A(2)” encysts in the gold-fish or Carassius auratus, and the encysted larvae develop into Exorchis maior Hasegawa in the intestine of Parasilurus asotus. 6) The above-mentioned differences between Cercaria A1 and Cercaria A3 in regards to the morphological and biological data prove the fact that the so-called “Large egg type of Metagonimus” is quite different from M. yokogawai.