As is well known Echinostoma cinetorchis is found occasionally as a parasitic worm in the human body, but it has been left unclarified whether it has some pathogenicity or not. Of course there isn't any report concerning the pathologic changes which may be caused by this parasite. This is the reason why the present author has carried out this experimental studies on the pathogenicity of the worm. The results obtained through the observation on rats are summarized as follows: 1. The experiment of feeding of the metacercaria of Echinostoma cinetorchis proved the worms living on the surface of the mucous membrane mainly in the lower parts of the small bowels. 2. The hosts carring the worms were attacked by slimy diarrhea followed by the symptoms such as nutritional disturbances and delated growth. 3. The intestinal walls, on which the worms were found to be adhered, gave a series of pathologic change, i. e. localized defects or focal necrosis of the mucous membrane, which generally appeared against the suckers of the worms, atrophy or degeneration of the mucous membrane surrounding the worms which appeared sometimes on the fairly large area around the massed worms and catarrhalic inflammation on the adjacent mucous membrane. Peyer's patches and mesenterial lymphnodes showed a slight reactive inflammation, hypertophy of follicles and lymphadenitis of simple type, rarely accompaning the circumscribed necrotic area. 4. The grade of clinical and pathological findings mentioned above changes according to the number of the worms attacking, i.e. the attack by the worms below ten in number gave not so marked injuries to the host, however, a severe pathologic change was caused by the attack of worms more than about thirty, which resulted frequently in the death of young rats. 5. Beside the attack on the intestine this worm can invide into the liver and cause a certain pathologic change which will be introduced precisely in the following paper. 6. Endemiological researches done in the districts of Takaya-cho, Ibara-shi of Okayama-ken revealed that the intermediate host of this worm is a kind of shell fish, viviparus malleatus, 93% of which were found to be carring the metacercaria of this worm. Thus it has been clarified that the endemic disease, Echinostomiasis, found in the said districts is caused by taking the low or half-boiled viviparus carring the metacercaria.