The degradation of tributyltin (TBT) and changes of bacterial number and community structures were investigated in microcosms using the sediment collected from the Mekong River, Vietnam. Concentrations of TBT in sediments were less than 0.62 ng/g (dry wt), lower than those reported from other areas. TBT-resistant bacteria were found in the three sampling sites, and the occurrence rates were 11-16% out of the total viable count. In this microcosm experiment, initial concentration of TBT [1.0-1.4 microg/g (dry wt)] decreased to 0.6 microg/g (dry wt) during 150 days, whereas that in the control microcosm with autoclaved sediment did not change, indicating that Mekong River sediment contains high TBT-degrading activity by microorganisms. The occurrence of TBT-resistant bacteria and the bacterial community structures monitored by denaturing gradient gel electrophoresis were almost the same between test and control groups, indicating that the addition of TBT had little influence on microbial community structure. Mekong River sediment seems to have a stable microbial community against TBT pollution.