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It is well established that guinea pig, rabbit and rat are equally susceptible to experimental infection with the Chinese liver fluke, Clonorchis sinensis. However, little work has been done on the studies of the host-parasite relationships between the liver fluke and mouse host. The experiments were undertaken to investigate the susceptibility, development and sexual maturity of Clonorchis sinensis in mouse host. The metacercariae of C. sinensis used in these experiments were isolated from the fish, Pseudorasbora parva by digestion technic. And in all these studies the mice weighing around 20 g were commercially purchased and infected with the metacercariae under slightly narcotized condition with ether. The animals were starved overnight before infection. The mature metacercariae, e.g. 5, 10, 20, 30, 50, 200 were given orally into stomach by means of the polyethylene tubing (intramedic, Clay Adams, Inc.PE-90/S12) respectively to each mouse of groups ranging 4 to l5. After various period of infection, the mouse liver was first opened along the common bile duct and cut into small pieces for collecting and counting the worms. For the histopathological examination of the liver, the tissue specimens were fixed in formalin, sectioned and stained with haematoxylin-eosin. Another experiment was separately set up for the study of egg laying capacity of C. sinensis in mice. The following results were obtained: 1)Mice were successfully infected with metacercariae of C. sinensis in all cases of the groups except the group given 5 and 10 metacercariae, in which the infection was not established in one case of each group. Therefore it is assumed that mice are susceptible to infection with this worm. In an earlier period ot infection, the worms were found mostly in common bile duct and intrahepatic biliary passages with same rate, however in later period, they were seen rather in the intrahepatic biliary passages (73 %) than in common bile duct and gall bladder. The recovery rate of fully matured adult worms (less than 10 %) was lower than that of the average recovery rate of the worms in general (22.3 %). 2)Under same age of infection or even in same host, the size of collected worms showed the great variations. Oral sucker was smaller in size than ventral sucker in the early stage of infection. After 11 days of infection it reversed. The posterior part of body length began to elongate since 5 days after infection, and therefore the ratio of antero-posterior part became l to 3. Fully matured adult worms were only collected after 30 days of infection. The first positive appearance of eggs in feces was on the 17th and 18th day after infection. However, the egg-laying capacity in mouse host seemed to be stabilized since 30 days after infection. 3)Histopathologically, the cystic dilatation of medium to small biliary passages was noted and focal but diffusely scattered necrosis of the liver cells with scarce inflammatory cells was also observed as well as the hepatocellular degeneration, diffuse vascular congestions and adenomatous proliferationof bile duct.