Autoradiographic study was performed in order to know the distribution of exogenous radioactive substance, (3)H-6-Thymidine, by liver fluke, Fasciola hepatica. The worms were maintained in vitro for 1 hour, at 37 degrees C, in Tyrode solution containing (3)H-6-Thymidine. After the incubation, the worms were sectioned and autoradiographed by applying the techniques of microautoradiography. The black grains derived from labeled substance were highly accumulated in the vitelline duct and vitelline glands. Moderate amount of blackening showed in the subcuticular muscle fibers, testes, ovary, cirrus sac, oral sucker and pharynx. Slightly labeled particles were in the reticular tissue, ventral sucker, intestinal epithelium, prostate, uterus and ova. There can hardly be found the black grains in the cuticle.
