In order to study host-parasite relationship in trematode, the susceptibility in rabbits of Clonorchis sinensis were examined. A certain number of metacercariae of C. sinensis were administered orally to rabbits, and rats and dogs were used to compare with rabbits. the results were summerized as follows: The rabbits were successfully infected with metacercariae of C. sinensis in all case of the groups given 10 to 500 each. But the highest recovery rate of the worm was 33.0 percent in the case given 100 metacercariae. In the early stage of infection, the worms were hardly found, but 10 days after the infection, the number of worms were detected easily. Three months after the infection, the recovery rate of worms was 31.3 percent in the rabbits given 300 metacercariae. The recovery rates of the worms in two months after the infection were 27.4 percent in rabbits, 51.0 percent in rats and 54.6 percent in dogs. Morphologically the body length and the body width markedly increased in the period until 10 days after the infection, and after that the genital organs began to get mature gradually while the growth of the body showed down. The measurement of adult C. sinensis recovered from the animals two months after the infection was the largest in rabbits followed by rats and the worms from dogs showed the smallest measurement.
