Metacercariae of Microphalloides japonicus were isolated from five kinds of crab which were collected at the Delta areas of Naktong river. These metacercariae were fed to experimental animals. Some of these metacercariae were also incubated in screw cap tube at 37 degrees C. The longevity and developmental difference between flukes grown in vivo and in vitro were described. 1)The total number of crabs examined were 2245 and Helice tridens tridens showed highest incidence among five species of brackish water crabs. 2)Some of these metacercariae were fed experimentally to mice and rats, and the mature worms were obtained in the host intestine 24, 48, and 72 hours after infection. The adult and larval morphology of this fluke was described. 3)Utilizing three kinds of medium, the adult worms of Microphalloides japonicus have been successfully developed under axenic conditions in vitro. Eggs containing adult worm were obtained in all of three media, i.e., Ringer's solution, Tyrode's solution, and Serum-Tyrode (1:1) solution. The adult worms were morphologically similar to flukes grown in vivo, but there were certain degree of difference between adult flukes grown in vivo and in vitro.