A total of 220 tadpoles was captured in 6 areas and total 117 frogs, Rana nigromaculata, were collected in 11 areas in Korea. They were examined for their infecion status by the metacercariae of Fibricola seoulensis by peptic digestion technique and by histological observation with hematoxylin-eosin staining. This study was carried out from August 1983 to September 1984. Followings are the results. The tadpoles of R. nigromaculata were positive for the metacercariae from 3.3 percent to 100 percent by area. The number of metacercariae per infected tadpole ranged from 1 to 584, and the mean number per tadpole ranged from 7.6 to 221 by area. The metacercariae from 16 tadpoles were counted by the body portion. A great majority of the metacercariae was collected from abdominal cavity, 98.3 percent of 484 counted larvae. And 6(1.2 percent) larvae were from proximal tail and 2(0.4 percent) from trunk. Histological sections of tadpoles showed many metacercariae in abdominal cavity but none in other parts. The larvae were free in the spaces among intestinal loops or around primitive liver. A few larvae were in duct-like tissues near trunk wall. There was little infiltration of inflammatory cells. The metacercarial infection rates of frogs ranged from 0 percent to 100 percent by area. The larval burden was 1 to 470 by infected frogs, and mean number ranged from 1 to 175.6 by area. By above results, it is suggested that the cercariae of F. seoulensis may infect R. nigromaculata already in the stage of tadpole. Almost all of the metacercariae were concentrated in abdominal cavity of tadpoles. According to the infection status of frogs, this fluke is prevalent almost nation-widely in rice paddies in Korea.