A comparative study of Bacteroides fragilis and E. coli bacterial infection in the biliary tract in relation to the pathogenesis of pigment stone formation was carried out on the basis of gallstone rabbit's model of anaerobic bacterial infection. One hundred and twenty Japanese hybrid big-ear white rabbits were randomly divided into four groups: 14 in control group, 31 in B. fragilis (BF) group, 42 in E. coli group and 33 in the mixed group. In the experimental groups we successfully made gallstone formation in aerobic, anaerobic and mixed bacterial infections in biliary tracts respectively. On 7, 15 and 30 postoperative days the survival rabbits were sacrificed for investigations. Our experiments demonstrated that the incidence and amount of stone formation in the mixed group were the highest among the experiment groups. The key point to preclude stone formation was to control the bacterial infection in the biliary tract as early as possible. The results suggested that the ability of production of beta-glucuronidase in BF group was significantly higher than that in E. coli group. The author considered that BF was more important than E. coli in the pathogenesis of calcium bilirubinate gallstone formation.