The echinostomatid metacercariae encysted in the gill of the fresh water fish, Pseudorasbora parva were identified through obtaining adult worms after experimental infection to mice. In addition, a brief course of worm development and maturation was observed in this experimental host. The results were as follows: The echinostomatid metacercariae were elliptical, golden yellow, 0.073-0.078 mm long and 0.054-0.065 mm wide. Their head portions were characterized by the presence of a head crown armed with collar spines of total 24 in number and interrupted at the mid-dorsal side of the oral sucker. The average rate of worm recovery from 12 mice (on the 1-21th postinfection days) was 19.4 percent and the rate revealed no decrease in accordance with the increase of infection duration. The worms were collected chiefly from the lower part of the small intestine. After the infection, their sexual maturation was attained in 5 days and their growth in size nearly completed in 7 days. The early growth curve of genital organs was S shape while that of nongenital organs was C form. In 5 day old worms, l or 2 eggs were found from their uteri and the stools of mice revealed echinostomatid eggs from the 5-6th postinfection day. The 7 day old adult worms were ovoid in shape, 0.54-0.69 mm long and 0.29-0.34 mm wide, and characterized by a well developed head crown with 24 collar spines and vitelline follicles distributed from the acetabular level down to the posterior end of body. Based on these characters they were identified to be Echinochasmus japonicus Tanabe, 1926. From these results, it is verified that P. parva is one of the second intermediate hosts of E. japonicus in Korea.
